There's a theory that Wobbuffet's blue body is a decoy, and that the black tail is the actual Pokemon. Many believe this due to the fact that Wobbuffet can take such a beating and not be affected. Multiple Pokedex entries also say that Wobbuffet is very protective of its tail. The Pokedex entry in Pokemon Silver claims "To keep its pitch-black tail hidden, it lives quietly in the darkness." In Pokemon Diamond, Pearl, and Platinum, the Pokedex entries claim "It desperately tries to keep its black tail hidden. It is said to be proof the tail hides a secret." On top of all of that, the tail does possess a pair of eyes, which further supports this theory.