Not necessarily. You could load up one black hole with electrons and the other black hole with anti-protons ... both would have negative charge, -Q ... but the latter would have a much greater mass, since anti-protons are much heavier than electrons, despite having equal charges.
"Antimatter" is really only "anti" in a few respects. First, the lepton or baryon number has the sign flipped. An electron has a positive lepton number; a positron would have a negative lepton number. The charges are also flipped, where applicable (anti-neutrons still have a charge of zero). Isospin is flipped; spin is not. Mass is not, in three ways.
First, if you converted an anti-proton into pure energy (ignoring charge conservation, et al), it is the same energy as you would get out of a proton.
Second, an anti-proton exerts the same gravitational pull as a proton. A world of anti-matter would cause you to fall toward it the same way the Earth would.
Third, a force exerted on an antiproton causes it to accelerate along the same vector as the force, so the inertia is the same as regular matter.
People have theorized, myself included, on what some kind of matter where mass was negative (let's dub it nega-matter) would look like, which is to say that a force pushing it away would contrarily draw it toward you. You can simulate it in particle life, likely, but what would happens is that all of the nega-electrons would feel electrostatic repulsion and therefore clump together, as would all of the nega-protons. These two lumps of enormous charge (we just don't get that in real life, electrostatics are dozens of orders of magnitude more powerful than gravity) would create an "attractive" field between them of nearly incalculable strength and, because they would move opposite to the vector of the force, promptly zoom away at many nines of the speed of light. In short, they would sort themselves out and fly away to distant corners of the universe with great haste.
> would be indistinguishable from a black hole fed entirely by the equivalent matter
> one black hole with electrons and the other black hole with anti-protons
But electrons and anti-protons are not equivalent. Positrons are.
That's what I meant -- if the same energy of electrons and positrons are used, the black holes would not be indistinguishable, their charges would be different.
