It seems that, back in the 20s/30s, record companies wanted to market black singers in a narrow way i.e. blues singers, and so Son House, for example, is now thought of as a blues singer whereas Leadbelly, who was discovered by a folklorist, is considered to be a folksinger who just happened to sing a few blues. Alan Lomax, in his fabulous, " The Land Where the Blues Began" lists Muddy Waters repetoire from the early 40s and it contained all sorts of different songs.