From Creatures Wikia
The Black Hole is a bug in Creatures 1 and Creatures 2, caused by improperly defined rooms and spaces between rooms. These spaces can have objects dropped into them, and Norns can walk into them. They are named for their habit of making things disappear, not their colouration - they are invisible. If you lose a Norn down a black hole, quickly export it before the game autosaves. If you lose an object down a black hole, it's gone for good. Some objects (like eggs) can eventually come back from Black Hole to Albia, but it's very rare. Black holes can be fixed using the Black Hole Patch by Slink.