[syhprum (OP)]

The physics of Colliding black Holes has been well explored by computer simulations and various video clips are available.
Could a blackhole form by the collapse of a massive antimatter star? I see no reason why not so the interesting case arises what would be the result of the merger of such a (anti)blackhole with a normall one ?
I realise the formation of such a blackhole in our universe is all but impossible but within a computer all things are possible !

[Pikaia]

Antimatter is affected by gravity in exactly the same way as ordinary matter, so they can collapse into black holes, in theory. The only problem is that there are no antimatter stars.

A BH is described entirely by its mass, spin and electric charge, so a BH formed from collapsing anti-matter would be indistinguishable from a normal matter BH.

[JMLCarter]

There are no KNOWN antimatter stars. Neither is the assymetry of the universe, i.e. the preference for matter over anti-matter fully explained. However, there is much still to learn about the universe, and the observable universe is just part of that. There is much unexplained about the observable universe.

Matter and anti-matter behave the same way in relation to gravity. Black holes comprising each would attract in the same way as two matter based black holes. When matter and anti-matter meet in normal space they annihilate to release (a lot of) energy according to E=mc². However, should such annihilation occur within a black hole event horizon even this energy release would be unable to escape the event horizon. Therefore sadly the collision of two black holes is likely to be somewhat unspectacular compared to a hypothetical massive matter/anti-matter collision (although the gravitational disturbances will shake things up).

No-one has observed inside a black hole, there is a postulated a singularity in the center, about which we cannot predict any laws. It's a point of infinite density and we just have no way to get a handle on that. There is no way to know what happens to matter (or antimatter) when it enters a black hole, and whether it meaningfully persists for any length of time beyond the event horizon.