The most common cause of a dark toe nail is due to a subungual hematoma, or bleeding under the nail, according to WebMD. This is often due to the result of a crushing injury to the tip of the toe.
The discoloration of the toe nail is caused by blood pooling underneath the nail itself. A subungual hematoma should grow out with the nail. If it does not, or there is no history of a crushing injury to the toe, it should be evaluated by a health care provider. If the pain needs to be relieved, a health care provider may perform trephination, or decompression of the pressure under the nail, explains WebMD.