The act of checking your surroundings for any black people prior to making a potentially insulting racist joke and getting beaten. Usually the joker will state they are making the check out loud, much to the hilarity of those familiar with the term.
Johnny: So this black guy walks into a bar... Wait, gotta do a blackground check....
Tim: Oh, that. LOL.
Johnny: Nope, no can do, there's one over there.