Once a good friend did that to me.
I was quite surprised, and wondered if he had had too much brandy (or not enough?) that night. Anyway, I couldn't talk to him to ask him why, but eventually he unblocked me as if nothing happened....I was actually in tears the moment it happened, so I really didn't think being blocked would have affected me like that. When angry people do it, I just figure it's a personality conflict. But when a friend does it, it hurts.