This is one of the hardest things to detect when potatoes are brought in from the field and sorted to go into bags for consumers.  Black spots just below the skin of the potato can occur if the potato is stored too cold (below 40 degrees) or when a potato is dropped more than 6 inches or something heavy is placed on top of them.  The damage does not appear immediately but can become noticeable after one or two days in storage.  Since the skin is not broken it is very hard to detect black spots until the potato is cooked.

Despite their hardy appearance, potatoes can bruise as easily as a banana or apple.  While it might not be convenient, you can still eat the potato as long as you trim away the black spots before cooking.