Al was not making fun of Blacks by doing Blackface. You might be surprised to know that Al did a lot to promote Black singers when they weren't in vogue. Check out his wiki and judge for yourself.
Al Jolson was hip. Of course I am not cool with the blackface make-up routines. You can't really hate though since it was hip for it's time just as making fun of white people seems to be hip for our time.