Get the finger in ice water. If the accumulation of blood is under the nail (subungual hematoma), then it may need to be drained. A hole is bored through the nail (not as bad as it sounds) to allow the blood to be released. The pressure of the accumulated blood is painful, so this procedure relieves a lot of pain. If the blood accumulation is in the soft tissue of the finger, it cannot be drained...it will need to be slowly reabsorbed by the body. If you think the injury could of resulted in a fracture, only an x-ray could confirm this.
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