The Greatest Entertainer who ever lived was Al Jolson.
He didn't 'black up' to demean anybody. It was an accepted art form and he is the only man who actually brought it into the 'mainstream' from Vaudeville.
Black Americans were some of his biggest fans. Cab Calloway co-starred with Jolson in "The Singing Kid" in 1936, and yes, they did a number together with all black performers and Al - in blackface.
Calloway was also one of the original members of the Al Jolson Society.
Back in 1985 I attended a screening of "Halellujah I'm a Bum" (a 1933 Jolson film that is probably his best, with a Rogers and Hart score and available on DVD) at the Smithsonian Institution in DC for Jolson's Centennary.
In the very back row of the theater there were about 10 finely dressed elderly black ladies, the youngest of whom was probably 80.
They didn't dress up like that to go see a star who 'demeaned' anybody. They came to remember a local Baltimore boy who made good, and whose fans they had been in their youth.
If you want more information about the greatest singer America has ever produced, I invite you to go here:
http://www.jolson.org