An injury to the toenail with blood trapped underneath is usually caused by a direct hit or pressure on the nail. This collected blood is called a subungual hematoma. It can cause throbbing pain, swelling, and a dark red, blue, or black discolouration under the nail. In many mild cases, the blood gradually dries, and the nail heals on its own over a few weeks to months. You should keep the toe clean, avoid tight footwear, and rest the foot as much as possible. Applying an ice pack wrapped in a cloth for 15-20 minutes at a time during the first 24-48 hours can help reduce pain and swelling. Keeping the foot slightly elevated may also give relief. Do not try to pierce or remove the nail yourself at home, as this can increase the risk of infection. If the pain is severe, the nail becomes very swollen, or more than half of the nail has turned dark, you should see a doctor. Sometimes the trapped blood needs to be drained safely using a small medical procedure to reduce pressure and pain. Medical attention is also important if the toe looks bent, there is difficulty walking, pus or fever develops, or the nail starts separating completely, as there could be a fracture or infection. In some cases, the damaged nail may fall off later, but a new nail usually grows back slowly over several months.