A reason given by one blind bluesman-- sorry I don't recall who-- was that "itinerant musician" was one career open to blind people. (The much-earlier Irish harper O'Carolan comes to mind.) So there were many who went into the traveling gospel circuit in the shouth, going from church to church, receiving bed and board if not cash payment, and practical help along the way. The ones who "made it big" could eventually afford to employ a partner for travel and sometimes the music (Ray Charles and Doc Watson come to mind), to facilitate the travel details, guide the blind musician onto the stage, etc.

("Blind" as part of the name, of course, was just ONE part of the tradition of naming bluesmen, as has been discussed in other threads on the blues.)

So "why there were so many" is about what economic opportunities were open to (A)people of color (B)who were blind-- two tough strikes in the time period that gave rise to the blues.

~Susan