Josh White, though not blind himself, apprenticed as a bluesman by acting as leader, or human seeing-eye-dog, to a series of blind singer/preachers starting in the early 20s when White was 8 years old. His masters included Blind Blake and maybe Blind Lemon Jefferson. A biography called "The Glory Road" by Dorothy Schainman Siegel tells the stroy. Good read, though pitched at a high-school readership.