If you hammered your finger & have the dark purple bleed trapped under the nail, there is a way to release the blood & save the nail.
It may hurt like heck, but not as much as it hurts now! put your finger on an ice cube to keep it cool & numb. Heat a paperclip end to red-hot & lightly touch the nail surface. Repeat 4-6x until you're through & you see the blood drop. Squeeze the blood out.