Why are so many clic blues players 'blind'?

Back in the early 20th century and late nineteenth century were the 1st and 2nd generations of free afro-americains.The only way of making a living for non educated blaks were mostly sharecropping or field work,which you can't do if you are blind.So a lot of your Early Black Blues guitarists played on the streets for pocket change, which in some cases became there means of making a living. All the above mentioned musicians and also Blind Lemon Jefferson started out this way .The lives of the early blues singers were full of many forms of social, psychological and spiritual pain — imprisonment, violence, tragedy, addiction, madness and a sense of damnation — but the pain of the blues manifested itself most identifiably for me in the blindness that afflicted many early blues singers; Blind Blake, Blind Lemon Jefferson, Blind Willie McTell, Blind Willie Johnson. Blindness seemed as much a part of the blues as lonesome freight trains. But blindness was not just a lyrical motif: it marked the bodies of blues artists like a sign indicating the authentic presence of "the blues".