ooo good question. Now a black hole of only antimatter could in theory exist, however, there is no such thing theorised because there is so little antimatter that it is not sufficient for it to form a black hole. A black hole of both matter and antimatter could not exist as the two would cancel each other out and annihilate.
Amazing question btw 🙂
If you worked out the mass of the black hole and poured the same amount of matter in, it would either be a black hole with twice the mass (if it’s a matter black hole), or it will annihilate and have no mass and no black hole anymore (if it’s an antimatter black hole). You’d have to wait a really long time though as it takes an infinite amount of time to do this experiment.