Blues is much older than bluegrass. Bluegrass didn't come into being until 1945 and incorporates influences from blues and jazz as well as commercial country and old-timey influences. I don't think the blues masters played bluegrass although they did play for square dances and things like that. I think I've read that, after the voice, the fiddle was really the first blues instrument, partly because of availability and partly because it could play all the notes "in the cracks."

I don't think we should confuse Monroe Brothers' or early string band music or even the Blue Grass Boys from 1941-44 with bluegrass music. And for what it's worth, Bill Monroe often said he really thought of himself as a blues singer.

      - Mark