I got a black spot once. It really scared me, because people mostly talked about getting red pin head spots, which I had seen before, but this was a bigger black spot. I took a few days off and it slowly faded away. The day after it first appeared, it seemed to even be a bit bigger, as though it was kind of slightly spreading out. After about a week, it was almost completely gone.
I really don’t know why it appeared, but I am guessing it might have something to do with the fact that I didn’t do a pre-workout hot wrap that day (just a guess). I haven’t had any of those black spots since, but I always make sure to “warm up” before exercising, whether it is with a hot wrap, hot shower, hot tub, or whatever.