She immediately told her customer that the dark line was likely a symptom of melanoma. Subungual melanoma (aka nail melanoma) ...

Dermatology Nursing. 2007;19(1):83-84. Athletes may develop sports-related subungual hematomas. Repeated trauma to the nail bed occurs each time the toe contacts the inside of the sneaker.

Runner's toe, medically termed subungual hematoma, occurs due to repeated trauma to the longest toe. This trauma causes bleeding beneath the nail, leading to the characteristic discoloration.