Less light in the retina, yes. But there are different types of "blind."

So at least some blind people would be unaffected by melatonin differences which means you should still be able to find blind schizophrenia cases if the melanonin hypothesis is true

No, they specifically only mentioned people that were "born blind." I was answering your question:

> Do blind people get less light?

Yes, in their retina. But if you want to be technical, blind people 'get' as much light as anyone else on the rest of their body.

Some people born blind will still get the same amount of melatonin as sighted people. Therefore you should still see born blind schitzos. But we do not. The melatonin hypothesis is bunk.