"I believe the song refers to the huge abattoirs in Chicago in the 19th and earlier 20th century when vast flocks of cattle were brought in by train from the Mid-West States. These slaughterhouses were largely staffed by black workers..."

Newspapers.com has 1,329 hits for "killing floor" "cattle" from 1880-1949.

"any contemporary Afro-American" I definitely like Buddy Guy's blues singing better than Robert Johnson's. I'd rather listen to Tracy Chapman sing "Gimme One Reason" than to Johnson, although I think he was an excellent singer. Agree that Jerron Paxton is talented.

"they are still far from the real thing" Skip James didn't sing like Charlie Patton, Patton didn't sing like Frank Stokes, Stokes didn't sing like Willie McTell, Jimmy Witherspoon didn't sing like any of them, Freddy King sang another way, etc. But the measure of Jack Bruce is whether he sang like... who?