The denser muscle thing is more complicated than that. There's a reason strongman competitions lean White. An interesting thing though - from my limited experience their pain response is different somehow. I've done a little MMA and talking with black sparring partners they would say things hurt a lot, but didn't seem to have the same response to the pain. Like it wouldn't register on their face the same way or get the same involuntary response, even though they would say it hurt like shit afterwards. I've heard they're easier than Whites to knock out if you hit on the side of the head, like around the temples I believe but I don't know how true that is.