A lot about the Big Bang is speculation, and evolving different theories.   My understanding, from what I have read,   is many theories speculate that a black hole and the big bang are/ were much different. The Big Bang is thought to have been a singularity extending through all space at a single instant (more like a white hole, if they exist). A black hole is a singularity extending through all time at a single point. With the Big bang, the space near the singularity is flat rather than being tightly curved with a black hole.

I don't believe researchers figured out what happens to the entropy of matter falling into a black hole, which involves quantum mechanics and strong gravity.