Once I saw on Ripleys Believe It Or Not, a martial artist who said there was a technique in which if you focus somehow, you could get hit very hard and it would hardly phase you. It showed people strike him in the neck, hit him in the chest/stomach with a stick, etc., and it didn't even make him flinch. (I don't know if it was a fake or not, but maybe that's what Blaine may have been using here).

OR, Blaine could have had some kind of bean bag surgically inserted to prevent him from being hurt by Kimbos punches.