If blood appears black or dark red, it means the blood has a low oxygen level, notes Dr. Anne Marie Helmenstine for About.com. Bleeding blood of this color by itself is not a cause for alarm since all blood in the veins has relatively low oxygen.
Significantly dark blood may be a sign the blood is no longer in circulation or the blood is in the periphery veins of the body. Blood turns a dark red or even black-looking color when it is deoxygenated, according to Dr. Helmenstine. Humans of both genders may experience dark-colored bleeding when a wound draws blood from a vein. Blood from veins contains the least amount of oxygen of any blood in the body, especially in areas like fingers and toes. It is not uncommon for a person to prick a finger or a mole and see thick dark blood.
It is normal for women who have occasional heavy menstrual cycles to experience the release of black-looking blood, explains WebMD. This happens when a blood clot breaks apart and is released after being out of the body's regular circulation for some time. If a woman experiences regular releases of heavy dark blood during menstrual cycles, medical attention may be required as this could suggest a blood disorder.