Bruised nails are common, and can be due to a number of different reasons. Nails bruise because blood vessels in the skin underneath the nail either burst due to pressure from an injury or chronic stress on the nail, or are severed by a cut in the skin underneath the nail. Injuries to the nail include a heavy object dropping on it, kicking a hard object, or getting the toe wedged in a door or other similar moving object. Sources of chronic stress include poorly fitting shoes, slow nail damage from long distance running, and activities that require excessive toe pressure, such as tasks that require extended pedal use. The skin underneath the nail can become cut by either a bone fragment that protrudes through the skin when the toe fractures, or by a sharp object puncturing the nail and skin together, such as when one cuts their nail too short and bleeding begins.
As blood pools under the nail plate, regardless of the cause, it will turn the color of the nail blue, purple, or eventually black. This color will stay in the nail until the new nail grows outward, where the discolored area will be pushed slowly towards the toe tip. The blood will be liquid initially, and then will eventually dry up into a powder.
Small bruises involving a quarter or less of the nail's total size are often left alone, and allowed to grow outward. Larger bruises may require removal of the nail to help drain the blood and prevent infection, and allow for inspection of the skin underneath the nail for any cuts that need to be repaired or bone fragments that need specialized treatment to avoid bone infection. The nail will eventually grow back, although sometimes the injury that caused the bleeding in the first place may be severe enough to damage the nail root cells, leading to a permanently thick or misshapen nail. Nail bruises that are painful must be evaluated by a podiatrist or other physician to ensure that a fracture or significant skin injury has not occurred, and any nail discoloration that does not grow out with the nail needs a similar evaluation to rule out nail fungus or melanoma of the nail. Nail fungus can easily spread to other nails, but is generally treatable. Melanoma of the nail, while uncommon, can be deadly if not identified early enough before the cancer spreads to other organs. It can look identical to a common bruise, and needs specialist medical evaluation to make a proper diagnosis.