Among the more painful minor injuries possible is a smashed finger where the injury affects the nail. Usually these injuries are not serious, but are incredibly painful. In some fingernail injuries, a portion of the nail is lifted off the nail bed. In this case, the injury is rather clearly visible, along with some amount of bleeding. Nail lacerations are also clearly visible and sometimes appear more serious than they are because of the blood and the laceration going through the nail and the nail bed, perhaps to the cuticle.
In your case, you probably have what is known as a subungual hematoma, essentially a collection of blood between the nail bed and the fingernail. The blue coloration and the pain you feel are both due to the pressure that builds up as a result of the blood collected under the nail. If only a small portion of the nail has turned blue and the pain is mild, nothing needs to be done. In case of a more severe injury, the pain can usually be relieved by draining the blood from under the nail. Your family doctor can do this for you. If you find no improvement after this procedure, there may be a deeper, more serious injury, so you should go back to your doctor and ask him to suggest a course of action.
answered by G M