I had a tongue like that once, when I was pregnant. After a while the black bits became really sore and then fell off. I think it was probably down to lack of vitamins as the baby was taking all its nourishment and leaving me with nothing, plus I had severe morning, noon and night sickness. It eventually cleared up. The Doc just said it was geographic tongue. But as the others have said, you should probably go and get it checked out just to be sure.