What did babies eat before the advent of modern blenders?

Usually mashed food usually vegetables and fruits. I've personally seen babies being fed mashed potatoes and mashed banana.

Vegetables, fruits can be pureed. Meat is very easy to grind, or cooked meat can be chopped fine pieces. Carbohydrate sources like noodles, potatoes or rice are generally very easy to cook to a soft mash. Blender is not a must, even nowadays.

Methods for making fruit and vegetable purees existed long before the modern electric blender.

A mechanical food mill is usable on most cooked fruits and vegetables with very good results. I don't know about early historical times, but these things were very typical throughout the 20th century in locales where blenders were not common, for example in Eastern Europe.

Mothers would also use a special kind of grater, with four star-like pips on each hole, to create a slightly rough puree from either raw or cooked plants.

Then there are also meat grinders, also a post-industrial-revolution tool, but quite good at making a mash-like substance out of many foods.

Simple mashing also works remarkably well for many fruits and vegetables, especially when cooked. For that, you can use a levered press.

A more affordable and broadly represented instrument is the simple masher you already mentioned.

And to make it very simple, there is always the humble fork. A star chef might turn their nose up at a mashed puree full of 3 mm pieces, a baby would swallow.

And if you are looking for truly ancient tools, a mortar and pestle have been used for tasks like flour making thousands of years before mills were invented. They are not only usable with raw grains, but with most other foods, and are the preferred preparation method for many traditional recipes, even outside of baby food.

Of course, all of these tools require a much longer preparation time than using a blender. But spending that kind of time on food preparation was the norm, and people just did it.

I cannot give you data on which society used which tool in what proportion, or what was the actual ratio of using pureed foods versus foods which are soft for other reasons. But as you see, humans have always been able to puree food, independently of electric appliances.

Also, an aside which is not really related to history: if you want to puree a food nowadays, be it for a baby or for some other purpose, some of these tools are still superior to a blender, which is very sensitive about the liquid-to-solid ratio. You also have the choice of further modern tools such as food processors, slow juicers, and electric mill/grinders.

Mother's milk, overlapping with more solid food, was typically a major part of a baby's diet for much longer than we now think of in many western countries, where starting weaning at a few months and completing within another few months has become common in the last few decades, and breastfeeding is by no means guaranteed. Substitutes for breastmilk weren't as good or as readily available as now until the 20th century.

Babies develop physically very rapidly around the time they're ready to start weaning, so from around 6 months they can start getting some nutrition from properly solid foods rather than mash, especially if milk is still available. If however you're trying to replace milk with other foods from 4 months, babies will struggle with even soft solids.

The modern version of this is called baby-led weaning, essentially providing foods that the baby can eat with their fingers (this is actually becoming increasingly recommended by health authorities . Raw fruit and veg cut into grabbable pieces, stewed veg/meat, even bread (though that's rather salty). None of these are new - in fact some of them are among the earliest foods known to humans. Of course other primate species don't have blenders and wean onto things like ripe fruit (see First molar eruption, weaning, and life history in living wild chimpanzees, TM Smith et al.)

Further interesting reading:

• Isotopic evidence of weaning in hunter-gatherers from the late holocene in Lake Salitroso, Patagonia, Argentina. (a long paper, search for "weaning foods" for some discussion relevant to my first paragraph)
• From the ape's dilemma to the weanling's dilemma: early weaning and its evolutionary context, G.E.Kennedy
• Isotopes and new norms: Investigating the emergence of early modern U.K. breastfeeding practices at St. Nicholas Kirk, Aberdeen, K. Britton et al shows that for about the last thousand years weaning in the UK has commenced at a few months to a year and completed by about two years.
• Why are babies weaned early? Data from a prospective population based cohort study, CM Wright et al discusses modern practice and recommnedations in the UK.