How powerful was a black hole? To ordinary martialists, the power of a black hole couldn't be put into words. They just couldn't express it.
Absorbing a black hole that occupied one-third of a star system for cultivation purposes shouldn't be too much of a problem for Supremacies.
However, this sort of cultivation wouldn't provide Supremacies with much progress. Therefore, Supremacy-Level entities wouldn't waste their effort for nothing.
Universe-Grade powerhouses who were weaker than Supremacies didn't have the courage to do so. By the time one entered Universe-Grade, the various laws within the created internal universe had already started to stabilize.
It could be said that the controller of that internal universe was the Universe-Grade powerhouse themselves.