A blood blister underneath the nail usually causes a black toenail. The collection of blood under the nail discolours it and in some cases can cause pressure and pain. The toenail may or may not fall off.
A black toenail occurs when the toe becomes bruised due to repetitive bumping against the end of your shoes or sneakers. This can happen if you do a lot of downhill running or if your shoes are too small or too big. Usually, someone who has a second toe that is longer than the first toe (Morton’s foot type) is most susceptible to bruising his or her second toenail.
If there is pain due to the increased pressure, you need to drain the blood to remove the pressure. The sooner this is done the better. Normally it is best to have this done by your family doctor or podiatrist. However, to do it on your own, start by swabbing the toenail with alcohol. Next, take a paper clip, or another sharp narrow object, heat it in a flame, and then slowly push it through the toenail. Drain the blood and apply an antiseptic or antibiotic cream and cover with a bandage.
The best way to prevent black toenails is to ensure that you have proper fitting shoe gear. The toe box should be wide enough and the shoes long enough so that your toes don’t bump against the sides or end of the shoe. There should be approximately ½” (1.25 cm) between the longest toe (not necessarily the big toe) and the end of the shoe. Wearing blister-free socks might help prevent friction as well.
Usually symptomatic. If the toe throbs and is painful, it is best to take a few days off and let the toenail heal.