If there's a word which has a greater air of despair, dirt and death than blackdamp then it's probably chokedamp; which is the same thing.
Blackdamp is a gas that's lethally low in oxygen and high in nitrogen and carbon dioxide. It's found in mines, especially after an explosion.
You can also find it (though I very much hope you don't) in ships' holds, tunnels and sewers.
The symptoms of blackdamp are very much like those of tiredness, but the effects can progress to unconsciousness and death in a matter of seconds.
Now I come to think about it, blackdamp, a nasty-sounding word for a horrible thing, is a superbly evocative piece of language.
But it's one we must all hope never to have to use, all the same.
Word Not To Use Today: blackdamp. Also called stythe, as well as chokedamp. The damp bit is believed to come from the German Dampf, which means vapours.