I suspect there were many blind bluesmen because it was something they could earn a living at. If they were born blind, or had an accident, back then there wasn't much opportunity or support. But if you could perform, you could at least make a living of some sort.

And of course, it wsa a very different music industry back then. They'd be playing bars and roadhouses, relatively small places. Their fame came if they recorded, and if they recorded they got paid by the song and the record would fade with time, as did they. Only some got lasting fame. So there were probably many we never heard of. And many did "disappear" fading into a regular life until someone found their old records and sought them out in the early sixties. Mississippi John Hurt's career was long in the past after making that album, he'd moved on to other things. If the records had been all destroyed, he'd no longer exist as a musician. Robert Johnson, for all his fame and legend, had only one recording session, he may have had some level of fame back then, but his real fame came decades after he died.

It's later bluesmen, like Leadbelly, who did better fame wise since by then the recording industry had changed, so if you hit it big, you'd keep at it.