Why did European writers and artists continue to depict the Moors as being black skinned, even though they were mostly Berbers and Arabs

The key is the theoretical term "Imaginary." The imaginary is a post-structuralist influenced term which claims that cultures have, effectively, a collective subconscious. As you note the "imaginary" of the Muslim from North Africa stabilised on a black or dark depiction long before the triangle trade reformed Western European racism on an economic basis. The depiction is entirely unrelated to the reality of people in North Africa and entirely related to what lay in North Western European cultures.

We know from urban accounts in North Western Europe, and from Shakespeare, that "the Moor" or dark skinned people weren't viewed through triangle trade racism. So I'd suggest a combination of traditional saints depictions, "Africa" being distant and indirectly experienced, and an accumulated tradition casting "the Moor" as other.

I believe it was the exotic nature they were going for. Moor came to equate with foreign and negroid people were the most foreign looking at the time. There is also St Maurice who was depicted as Negroid from at least the 13th century. His depiction varies from Roman soldier to negro dressed as a knight. While the original St Maurice came from Thebes, Egypt, his name was likely conflated with Moorish during medieval times where the word later became synonymous with foreign.

You can also compare Morris Dancing where the word Morris is thought to derive from Moorish though this could be conflating the words due to similarity like Maurice. Morris dances often involved a blackface character so there could be another coincidental link there. The blackface character could share origins with the Zwarte Piete of The Netherlands.

The "Moors" were a very diverse group.

The first invading "Moors" were Arabs and Berbers from North Africa, the Berbers being descended from the Moors of Classical antiquity and thus probably mostly looking like other Mediterranean people.

A lot of native Spaniards converted to Islam over the centuries, and so a lot of "Moors" looked like native Spaniards and no different from Christian Spaniards.

Muslim invaders and later immigrants intermarried with native Spanish converts to Islam and with Christian and Jewish Spaniards.

Furthermore, rich Muslims including monarchs usually had a number of slave concubines, often imported from thousands of miles away, who usually became the mothers of their sons and heirs.

So if a Muslim monarch had a black African slave mother and a black African slave grandmother, he would be three quarters sub Saharan African and look a lot like a sub Saharan African.

And if a Muslim monarch had a pale skinned blonde northern slave mother and a pale skinned blonde northern slave grandmother, he would be three quarters northern and look a lot like a pale skinned blonde northern person.

Thus the Umayyad Caliph of Cordoba Abd ar-Rahmin III (889/91-961) whose mother was a Christian slave and grandmother was a Christian princess, had white skin, blue eyes, and light hair and beard, which he dyed black to look more Arab.

The movie The Long Ships (1964) features vikings vs Muslims. Sidney Poitier, an African-American, portrays a "Moorish" ruler of a land that might be in Spain and/or in Morocco, since it is not too far from the Pillars of Hercules (Gibraltar). There is a Scandinavian king Harald in the movie who I thought was supposed to be King Harald I the Fairhaired (reigned c.872-932) of Norway but is said in some sources to be King Harald I Bluetooth (r. c. 958/59-958/86) of Denmark.

If by that time an Arab ruler could look like a northern European as Caliph Abd ar-Rahmin III did another Muslim Arab or Berber ruler in that era could have looked like an African-American if he was descended from enough sub Saharan slave women. But I doubt if the filmmakers were thinking about that; they were probably influenced by the stereotype of "Moors" being very dark.

By this time Muslim armies were largely composed of foreign slaves and ex slaves. The foreign slaves and ex slaves in the Abbasid Caliph's armies in Iraq were mostly Turks from central Asia. In the case of Cordoba the slave and ex slave soldiers were 1) Slavs, some blonde and blue eyed, and 2) black Africans. Strife between the Slav and African soldiers contributed to the eventual collapse of the Umayyad Caliphate in Spain.

Thus by about 950 AD the Muslim leaders and warriors who interacted with Spanish Christians in peace and war could look like anything from light skinned blue eyed blond northern Europeans to black skinned, black eyed, and black haired sub Saharan Africans.

In the 11th Century the Muslim Taifa states in Spain called for help against the Christian kingdoms from the Almoravids of Morocco, who invaded and conquered the Muslim Taifa kingdoms and drove the Christians back to Northern Spain. When the Almoravid power in Spain decayed and new Taifa kingdoms emerged, the Christian kingdoms attacked them and the Taifas called for help from the new power in Morocco, the Almohads, who invaded, conquered the Muslim Taifas, and drove the Christians back to northern Spain.

Many warriors in the later invading Almoravid and Almohad armies were black Africans from south of the Sahara Desert.

And because the sub Saharan African Muslim warriors were the most exotic looking Muslim warriors they probably became stereotyped as the typical Muslim enemies. Some Muslim warriors in Spain looked just like Christian Spaniards, and some looked slightly foreign, and some looked more foreign, but the black African Muslim Warriors in Spain looked the most exotic and foreign, and so would have become the model for artists wanting to stereotype Muslim enemies as foreign and exotic.