According to Right Diagnosis, several things can cause toenail bruises, including coagulopathy, anticoagulant medications, infection and foot sprain. However, bruised nails are most often caused by any sort of trauma or accident where the nail bed or nail matrix sustains injury. Also called subungual hematoma or bleeding under the nail, toenail bruises are most often attributed to crush injuries such as dropping something on the nail bed, according to WebMD.
In addition to a bruised appearance, a toe that is affected by a subungual hematoma throbs with pain due to the pressure of the blood building up between the nail and the nail bed. In many cases, this build-up leads to the eventual loss of the nail. Other symptoms include swelling of the toe and pain or tenderness in the toe, according to WebMD.
Subungual hematomas normally do not require treatment or medical attention. The bruise will simply grow out with the nail. However, if the trauma was caused by a severe blow, medical attention may be required for broken skin and bone fractures, advises WebMD. Dark bruises that do not grow out may indicate disease within the nail bed itself, which always requires proper diagnosis by a medical professional.