Hey, Callie HERSELF didn't become the black hole. She made the Sun implode into the hole, like stars naturally do. Then Callie was probably sucked into it, being Dead and Doomed.

Thanks Tumblr, for forcing me to resize the first Gif so it didn’t freeze. 

But, when Calliope Conducts the Ending, she encircles herself in a sphere with the Space Symbol, and collapses it onto herself, tearing a Black Hole where she is and beginning to absorb the Green Sun. It’s only AFTER the entirety of it has been absorbed that the Black Hole increases dramatically in size and begins to tear everything apart.