Interactions that occur between black people, notably crime that is perpetrated by one black person against another.
This term originated during the race riots in the USA in the late 1960s. The earliest record I can find of it in print is this piece from The Chicago Daily Defender, March 1968:
"The violence of black man stabbing black man, mugging black man stomping black man, raping black woman. Black on black. And a black crime against a black gets cancelled out in the mind of a white precinct commander."