How does one get a permanent bruise under the fingernail?

I’ve seen lots of people, mostly men who work with their hands a lot, with what looks like a bruise just under the fingernail. Sometimes it’s the whole nail, sometimes it’s just part of it, but it doesn’t go away. Sometimes it’s purple-ish, and sometimes it’s just plain black.

I ask because I just pinched the crap out of my thumb, right on the nail… to the point where an hour and a half later I’m still getting sharp pains in the top half of my thumb and it hurts to the touch, and I was expecting a bruise under the nail where it got pinched, but there isn’t one. It pinched pretty freakin’ hard, so I wondered what would have to happen for the permanent bruise to occur… if that’s what it really is.