There are of course various causes of blindness but I don't think I have ever heard of bad liquor beeing the cause for blindness among blues singers. I believe that most if not all of them were blind before they took up music this being one way they could attempt to make a living.
One of the most common problems that singers suffered during prohibition was Jake Leg and Limberneck caused by drinking adulterated Jamaican Ginger. There have been articles printed on this problem and a vinyl album of songs related to it.

Hoot