Oftentimes, discoloration of the toenail occurs because of increased pressure to the front of the foot in tennis shoes for long periods of time while running. This black or dark discoloration is usually small amounts of blood that accumulate underneath the nail from constant jamming in the shoes. Usually the big toenail, the longest toe, or contracted toes receive the most pressure during running and develop blood under the nail. This is called a subungual hematoma or “runner’s toe.If the nail plate is 100% affected by blood under the nail, the nail can spontaneously loosen or fall off. If you see signs of loosening or lifting of the nail, it is important to follow up with a podiatrist for proper care.