That black spot is a blister filled with blood. The blood has dried, and I'm told it will be reabsorbed by my body.

Incidentally, if one happened to be trying to smoke pot with a lighter, and they weren't good with lighters, and repeated lighting attempts had caused a blister on their thumb, and they then managed to get high right at the exact moment that the blister on their thumb started filling with blood, it might be scary and disconcerting if such a one had never had a blister fill with blood before, and didn't know whether it required medical attention or not.

It doesn't require medical attention, it turns out.. at least this one didn't.