You misunderstand what a theory vs a fact is. If a fact is a lego brick,a theory is a what you build.

A theory collects facts and builds a cohesive narrative to explain why they are. Most importantly there will be gaps, so a theory makes predictions about facts that we don’t have yet. When a new finding fits expectations, then the theory is supported. When a new finding is unexpected or doesn’t fit, it’s time to reclaim the bricks and build a new structure.

So black holes were predicted and fit the theory very well. It increased our confidence that the theory was moving in the right direction.