Black Holes are objects so dense, that nothing within them can escape their strong gravity, not even light.
Quaric was selling black holes in the Netherworld in his shop. ("Perceptual Boundaries" by Xaq)
According to The Uzé Model, "probably just Flux bubbles that have their final destination in a void between Universes. There is no time in that space, since there is nothing in that space, and that spills to our Universe, distorting time in the process." (Chronicles Addendum: "The Uzé Model" by Compech Dev)