Bubba - I read an interview (I think with Frank Zappa)where he was asked about his favourite guitarist in various genres. He said his favourite slide player was Elmore James; "He played the same solo for every song, but I get the feeling he really meant it."
Wesley - I love John Hurt too, but don't really think of him as a blues singer. Like Lead Belly, he did some blues, but a lot of other stuff too.
I've just rediscovered Karen Dalton while organizing my vinyl, and though she performs in a variety of genres, she sure can sing the blues.