Shouldn't black holes masses add when they collide?

[gvntd2018]

I may be wrong, but I assumed when 2 black holes collide, the result is a black hole with a larger mass.  When they collide in the current Universe Sandbox, the result is one black hole explodes, its mass apparently disappears, or is converted with 100% efficiency to energy.

Or is the mass added and just the black hole's label isn't changed?

In any event, Universe Sandbox is a very interesting application.

[im4space]

We know about a few black hole collisions, from Wikipedia

https://en.wikipedia.org/wiki/Binary_black_hole

"a pair of black holes with estimated masses around 36 and 29 times that of the Sun spun into each other and merged to form a 62 solar mass black hole"

So I would agree with your assumption that when 2 black holes collide the result is a black hole with a larger mass.

[Physics_Hacker]

The labels (names) of black holes don't change when they collide. If you select the new black hole and click on the properties window you'll see though that their masses did add correctly, unless there is some weird bug going on.