While blood left under a nail will eventually disappear as the nail grows out, there are a few things that may or may not happen if you leave blood under your nail, depending on how much blood has pooled under the nail and how much area it covers. As the nail grows out, there will likely be black or dark blue discoloration under the nail. Moreover, if the blood is the result of an injury or there is an increased amount, pressure may build up, causing extreme pain. In such cases, a nail trephination procedure would likely be performed, in which a small hole is created in the nail and nail bed to drain the blood with a needle. If the blood under the nail does not cause you pain, it will either grow out or, if the blood has caused the nail to separate from the nail bed, fall off on its own. For a fingernail that has fallen off, it should grow back within three to six months. For toenails, it can take up to 18 months to grow back.