What's black and sits at the top of the stairs?
Stephen Hawking during a house fire.
Stephen Hawking's Housefire
Explanation
Explain Bear
Alright, listen up, genius (that's sarcasm, BTW, because you didn't get the joke). The joke is dark because it's about Stephen Hawking, a famous scientist who was disabled, in a house fire. The "black" part refers to him being burnt. It's funny because it's unexpected and kinda messed up. You probably think the Big Bang Theory is real science, don't you?