Have there been ruthless rulers who held on to power despite being blinded?

Theodore Komnenos Doukas Became ruler of Epirus in 1215 and conquered many lands including the Kingdom of Theassalonika. He was proclaimed emperor about about 1224 to 1227. He was captured and blinded by the Tsar of Bulgaria in 1230. His daughter Irene married the Tsar of Bulgaria in 1237 and Theodore was set free.

Ex-emperor Theodore ruled Thessalonika in the name of his sons until he was captured and imprisoned by Emperor John Duoukas Vatatzes in 1251 and died in 1253.

So Theodore was a blind person who ruled in the name of nominal rulers from 1237-1251 (though not until his death), and fits that part of the question. The ruthless part of the question is harder to decide although like most rulers of his time his reputation is not exactly saintly.

https://en.wikipedia.org/wiki/Theodore_Komnenos_Doukas[1]

Emperor Andronikos IV Palaiologos (1348-1385) became co-emperor in 1352 but was (partially?) blinded and imprisoned in 1373 as punishment for rebellion. He escaped in 1376 and made himself ruling Emperor from 1376 to 1379, throwing Emperors John VI (father) and Manuel II (brother) and ex emperor John VI Kantakuzenos (maternal grandfather) into prison. He was deposed in 1379 but in 1381 was made co emperor and heir to the throne again, and given Selymbria as his personal domain. Thus he was a blind person ruling a subordinate domain when he died.

His strife with his family contributed to speeding up the Turkish conquest of the Balkans and the eventual conquest of the Roman empire.

https://en.wikipedia.org/wiki/Andronikos_IV_Palaiologos[2]

His son John VII (1370-1408) was made co emperor in 1377 (when his great grandfather ex emperor john VI was still alive) but deposed with his father in 1379. John VII was (partially?) blinded in 1379 age 9, or possibly in 1373 age 3. He usurped the throne in 1390 and ruled for 7 months. He then ruled Selymbria as a vassal of his grandfather John V. During the ottoman siege of Constantinople from 1394 to 1402, Emperor Manuel II left in 1399 to seek assistance in Western Europe, leaving John VII in charge. John VII ruled Thessalonika from 1403-1408 as a vassal of Manuel II using the title of "Emperor of all Thessaly".

Thus John VII fits the blind ruler category, and usurping the throne in 1390 might be enough to put him in the ruthless category.

https://en.wikipedia.org/wiki/John_VII_Palaiologos[3]

Added 10-10-2022.

Enrico Dandolo becameblind in the 1170s and became Doge of Venice in 1192. And I guess his part in the capture of consantinople in 1204 makes him rather ruthless.

https://en.wikipedia.org/wiki/Enrico_Dandolo

Since you didn't specify bilateral blindness in the title, I'll advance both Philip II of Macedon and Antigonus I as being blind and massively ruthless.

Vasily II of Moscow. He was blinded by his enemies. According to Wikipedia his long rein was plagued by the greatest civil war in old Russian history.

tl;dr It would be good to distinguish between blind rulers in the cultural domain of European (post-Roman) Christianity and blind rulers elsewhere. There were an awful lot of blind European monarchs. This is due to the fact that rivals were often (very, very often) blinded to make them unfit to rule and occasionally ascended to the throne anyway.

On a different note, I agree strongly with @ed.hank that pre-modern rulers were probably always ruthless by the definition of the OP, because the OP bases the question on modern ethics that are anachronistic when applied to pre-modern times.

I will address the two issues one after the other.

Being blind or being blinded

In Christian Europe, rulers were seen as god's chosen leaders; they would be expected to serve as an example of flawless beauty. This is because according to the Bible, man was created by god in his own image (hence flawlessly beautiful) and the further you were from flawless beauty, the less godly you would be perceived as. Eyes, as a symbol of wisdom were particularly important.

For European Christian's obsession with flawlessness, compare also the veneration of virgin Mary. On some level, this veneration of beauty and flawlessness probably also had roots in ancient Greek art.

Now, if you were a contender to the throne in a medieval European country, you first and foremostly had to get rid of rivals. This included children and infants. A common way to do that without being perceived as a murderer of innocent children, was to have them blinded and placed in a monastery (so that they would 1. not have children of their own, 2. receive an education occupied with religious and politically unimportant matters, and 3. be well-guarded). This procedure was especially popular in the Byzantine empire. There are many examples for this, including some that were named in the other answers; see below.

Of course, often, the blinded successors to the thrones still had supporters and would at some point succeed to the throne anyway, if those supporters were victorious.

In non-European contexts, blindness did not have the same connotation and while blind people may have been perceived as physically impaired this would not be related to whether or not they were fit to rule. Hence, the blinding of contenders to the throne was much less commonplace (you would rather have them killed, castrated, or something like that); consequently less many successors to the throne were blind and there were less many blind monarchs.

Blind rulers in Christian Europe

Many have been mentioned in the OP or in various other answers: John of Bohemia, doge Enrico Dandolo, emperor Louis III (Louis the Blind), Béla II of Hungary, Tsar Simeon, Grand Prince Vasily II

There are many others: emperor Isaac Angelos, Simon Gurieli, Stefan Branković of Serbia, Prince Almos of Hungary (Béla II's father), Theodosius III of Abkhazia.

All of them were blinded on purpose, not due to natural causes. The list of politically mutilated figures in Byzantine politics on wikipedia has many more examples.

While this may suggest that removing rivals to the throne by blinding them was ineffective, since all cited examples eventually took the throne anyway, this is a sampling bias. It was very effective; the blinded usually never got back into politics again if they did not die of their injury directly. Consider some examples that did not return to the throne: Byzantine emperors Alexios Doukas, Leo Phokas, and Romanos Diogenes, king Magnus of Norway, king Bernhard of Italy, duke Boleslaus of Bohemia and Ilias of Moldavia. There are many more.

Blind rulers elsewhere

Elsewhere, blind rulers would generally be blind due to natural causes. This may include people who were born blind as well as people who became blind in their old age or illness.

An example for rulers who were blind when they were old is Tamerlan, the Mongol leader who almost destroyed the Ottomans half a century before they founded their empire. He led his army into his most briliant battle when he was already practically blind.

I struggle to find examples for rulers who were born blind. The ancient Indian king Dhritarashtra may be one example, but his historicity is disputed.

Ruthlessness

It is difficult to apply modern ethical interpretations to pre-modern contexts. The conditions you cite (mass murder, assassinations, execution of civilians, burning of witches, war crimes) were not generally seen as wrong (although some of them were at one time or the other). To give you some examples:

• Burning of witches at the stake was generally done according to lawful court proceedings and would have been seen as completely lawful.
• Conquering armies were generally accepted to be entitled to the spoils of war. In particular, they saw themselves as entitled to the spoils of war and history, you may have noticed, is written by the victors. That is not to say that tales of the cruelty of opponents (whether true or false) would not be used for political propaganda.
• There were no criminal investigations in the modern sense. The purpose of government was not primarily to be just but to maintain order. If some crime had happened, the most important thing was that someone would be punished for it. There were no medieval Sherlock-Holmes like figures going around towns striving to solve murder cases; instead they would round up some of the usual suspects and torture them until they confessed, then execute them. This was seen as completely lawful and just.

Note that this does not only concern which legal proceedings would be technically lawful. This also dominated contemporary perceptions of law and justice and the education of monarchs and citizens alike. They would not just follow these procedures, they would actually believe that that was right.

For details, you may want to have a look at analyses of pre-modern systems of law and governance, e.g. Michel Foucault's "Discipline and Punish", although this study (being the first in this field) is quite old and perhaps not quite up to date.

Whether monarchs came to be seen as ruthless more often than not had to do with who dominated the historiography about their reign. (And to what extend they pissed off the historiographers during their lifetime.) Take the Mongol invasions of Europe in the early 1240s as an example:

• From the European perspective they were seen as exceptionally cruel (and godless etc)
• From the Mongol perspective they saw themselves as fulfilling the destiny of the Great Khan who was the rightful ruler of the world. If Europeans resisted the Mongol armies, was it not their own fault if they were treated cruelly?
• From the perspective of Mongol commanders, it was all strategy. Be lenient towards the people you think you can work with in the future (including Christian princes like Alexander Nevsky). But set an example to everyone who will oppose you by not only being as cruel as possible, but also making sure that reports of that cruelty were disseminated as widely as possible. And by all means engage in respectful diplomacy with the Vatican.
• From the perspective of European leaders, it was all strategy too. Use reports of the cruelty of the Mongols for propaganda and to mobilize forces against them. Unless you want to work with the Mongols because you need them to participate in a crusade. And by all means engage in respectful diplomacy with the Great Khan.

One example might be An Lushan, a Chinese warlord who eventually became blinded by some kind of chronic eye problem. He rebelled against the Tang Empire in 755, and declared himself Emperor in 756. Aside from initiating a brutal war that led to mass civilian deaths, he also ordered the mass execution of Tang royal family members as well as the families of Tang imperial officials.

祿山怨慶宗死，乃取帝近屬自霍國長公主、諸王妃妾、子孫姻婿等百餘人害之，以祭慶宗。群臣從天子者，誅滅其宗。

Aggrieved by his son's death, Lushan ordered the Emperor's family - some 100 people including the Huo Princess, consorts, concubines, grandchildren and sons and daughters in laws - killed as sacrifice for his son. All the relatives of those court officials who fled with the Emperor were exterminated.

_{New Book of Tang, vol 225}

However, An Lushan murdered by another son a year later. He had regional authority for years before then, but he was only "in power" as monarch for a year. That was a family power struggle more than anything to do with his tyranny or blindness, though.

Another possible example is Béla the Blind of Hungary. While not really tyrannical, he does technically fit your requirement of "ordered the killing of innocent people". He was blinded in his youth, but managed to succeed to the throne of Hungary upon the death of his cousin Stephen II. Unfortunately, he turned out to be a bit of an alcoholic, which his courtiers exploited for personal gain.

The Chronicon Pictum records that, during one of his bouts of drunkenness, Béla had two men wrongly put to death.

It is to be feared, though, that it was during such a drunken stupor that he consented to the execution, based on false charges and without due court of law procedure, of two clergymen, "viros religiosos", named Poch, and Saul.

_{Kosztolnyik, Zoltán J. The Dynastic Policy of the Arpads, Géza I to Emery (1074-1204). No. 687. East European Monographs, 2006.}

Nonetheless, he held on to power until his death from natural causes.

One answer that I am aware of is

Erico Dandalo - Doge of Venice - was definitely blind

Emperor Manuel Comnenus "ordered his eyes to be blinded with glass; and his eyes were uninjured, but he saw nothing (1)

and I would consider him to be rather shrewd & ruthless. His avarice was responsible for the fall of Zara & Constantinople in the 4th crusade. First he put the crusaders in a rather bad position as they could not fully pay his fleet costs, so he used this to get the crusaders to attack and sack the cities.

The Venetians, under their aged and blind Doge Dandolo, would not let the crusaders leave without paying the full amount agreed to, originally 85,000 silver marks. The crusaders could only initially pay 35,000 silver marks. The Doge threatened to keep them interned unless full payment was made so a further 14,000 marks was collected, and that only by reducing the crusaders to extreme poverty. (3)

The crusaders sacked Constantinople for three days, during which many ancient Greco-Roman and medieval Byzantine works of art were stolen or ruined. Many of the civilian population of the city were killed and their property looted. Despite the threat of excommunication, the crusaders destroyed, defiled and looted the city's churches and monasteries (3)

Dandalo had an extreme hatred against the Byzantines and he did all he could to cause them harm. The Byzantine chronicler Nicetas Choniates wrote Donadolo "considered it nothing less than a death sentence not to exact vengeance on the Romans for their violence towards his people"

He held on to his power (even while blind) until the day he died.

Dandolo himself, exhausted from the campaign in 1205, died in Constantinople, comfortably in bed. (2)

sources:

(1) https://en.wikipedia.org/wiki/Enrico_Dandolo

(2) http://www.allempires.com/article/index.php?q=enrico_dandolo

(3) https://en.wikipedia.org/wiki/Fourth_Crusade