If you look closely, you can see that there is a "cushion" to absorb the shock, it is just very hard to see because of the shape and because it matches his skin color; pay attention to the sound it makes when Kimbo hits him, it doesn't sound like a punch over a human plus, why do you think David only lift up his shirt a bit and do not remove it totally?
Thus said, I cannot explain many of Blaine or Chris Angel tricks, these guys are very talented for street magic. However, I found out how they did a few of them by watching closely to their video, a little bit like the famous "walk on water" thing.