A "bluesman" is a musician who plays and sings the blues. But not everyone who sings the blues is a true bluesman. For example, Eric Clapton is a legendary blues musician, but nobody ever calls him a "bluesman".
Because the true bluesman doesn't just play the blues - he lives the blues. A bluesman starts out dirt poor and, if he's lucky, winds up larger than life. He does iconic things like name his guitar after a woman he never had, make a deal with the Devil, or get poisoned by a jealous lover.
The bluesman is a vessel for something deeper than his own personal experience. He takes great joy in music, but he also never loses a core of sadness, and even rage. A bluesman is a storyteller, but all of his stories are true.