Nail melanoma is officially called subungual melanoma if it is under the nail, or periungual melanoma if it's around the nail - rare, but potentially deadly. Half of all melanoma cases occur in those of Asian decent, 35% in darker-skinned people, and just 2.5% in white individuals.
One of the reasons why this is so serious is because more often than not, patients do not receive an accurate diagnosis right away. In fact, more often than not, the average delay of diagnosis for subungual melanoma patients is two and a half years, and it's not because people don't notice the black mark, but rather because it is usually attributed to something else.
In addition, a subungual melanoma looks like something called a subungual hematoma, or a 'blood blister. This is usually caused by some kind of trauma, which may be caused by slamming your finger in the car door. This will cause the tiny blood vessels underneath your nail to rupture, causing blood to leak out and discolor the nail.
In this instance, however, a new, healthy nail will begin to grow in and the discolored nail will grow out. The pattern of discoloration also tends to be more irregular, with puddles or drops, while with subungual melanoma, it develops as an organized band, arising in the root of the nail and extending to its edge.
So, if you notice a new, dark mark underneath any of your fingernails or toenails, and can't remember any kind of trauma that could have caused it. Your doctor will probably do a biopsy to check for melanoma. The good news is that if you catch it early, surgical removal can get rid of any of the cancerous cells before they're able to spread.