Black Holes have certain properties (mass, spin, charge, etc.). Anything that enters a black hole (matter, antimatter, whatever) is broken down into these attributes. Thus, antimatter affect the charge of a black hole, or its spin, but not its mass. When antimatter enters a black hole, it is no longer antimatter as we know it (same w/ matter). The truth is, we don`t know what happens to whatever goes in there, but the mathematics suggests that the only "information" retained is whatever properties the black hole has.