A blackhole is already the result of over filling. A blackhole is to mass as the rage virus is to people; throwing more at it just makes it more dangerous. However, unlike zombies, blackholes do eat each other.
The more matter that falls into a blackhole, the bigger the blackhole becomes. For example; the blackhole at the center of our galaxy (Sagittarius A*) has a mass of about 4 million suns, which is already the size of some small galaxies. Small globular clusters anyway.
If all the matter in the universe were chucked into the same giant blackhole you’d have: a really giant blackhole.