Keep the ice on it for a bit, otherwise, what Bill said.

The nail turns black because there is blood under it. If there's too much blood, it will push the nail off the nail bed. Ice would help reduce the bleeding and the swelling. A doctor might want to make a hole in the nail to allow the pressure from the blood to escape. I've never had that done and it sounds very painful, but everyone I've known who had it done said they were glad they did it and their finger felt much better after.