What happens if a black hole collides with a star?
It sort of can't happen.
Once a black hole forms all matter inside the event horizon has been sucked into the singularity (the actual point of the black hole), everything outside that goes into orbit around the black hole in what starts as an accretion disc and eventually becomes a quasar. So if it is a black hole, then it won't collide with a star but will pull that star into orbit around it.
A Quasar is a very large, very bright accretion disc that has all the mass that has gone into orbit around the black hole. This will include a large number of stars and planets, and they are the brightest objects found in the universe (in the order of 1,000,000,000,000 times brighter than our sun).