It could be "The Men Who Murdered Mohammed" (1958), by Alfred Bester. I have no exact recollection of the plot, but it goes like this according to wikipedia:
When mad scientist Henry Hassel discovers his wife in the midst of committing adultery, he builds a time machine with the intention of killing his wife's grandparents in their youth, so that she will never have existed. When he returns to the present, however, nothing has changed. In a desperate attempt to alter history, Hassel begins killing historical figures of greater and greater significance, only to learn that the nature of time is very different from what he had thought.
It doesn't match the girl protagonist neither the black hole part, but it's the "spaghettification" part that definitely rang a bell to me (though I may be in the wrong):
"We're like millions of strands of spaghetti in the same pot. No time traveler can ever meet another time traveler in the past or the future. Each of us must travel up or down his own strand alone."
"But we're meeting each other now."
"We're no longer time travelers, Henry. We've become the spaghetti sauce."