A nail that looks black or blue most often is one that has a subungual hematoma. That means that there is a collection of blood under the nail itself. This is caused by trauma to the toe (stubbing it, dropping something on it) and can also be seen from microtrauma (pressure on the nails from tight shoes or with runners.  If there is a black spot on the nail, it can sometimes mean the presence of a skin cancer or melanoma. An evaluation of the nail is important in either case, especially if the color change is spreading or if there is pain associated.