Black toenails, or subungual hematomas, occur when the toe has suffered trauma or injury. Examples include stubbing the toe, dropping something on the toe or crush injuries. These types of incidents will cause bruising as blood collects under the nail. Runners, especially those who get higher mileage, are more likely to suffer from black toenails because of the repeated friction of the big toe (or second toe) against the front and top of the toe box. For that reason, runners are advised to purchase training shoes that are 1/2 to a full size larger than their normal dress or casual shoes. The bruised toenails can be tender and even painful. However, treatment is only required if there is an underlying joint deformity or other type of tissue damage that needs attention.