Sore Tongue?

If your tongue is sore after playing (or practicing tongue blocking if it's a new technique for you) then you're pushing too hard. If you're pushing hard to block the holes needed, then use more of the top of the tongue (wider surface). Tip the face of the harmonica down slightly and bring the harmonica in your mouth a little bit more (tip of the tongue goes on the bottom cover plate). The wider surface also glides on the harmonica better.