Injured blood does not always turn black after a while. After an injury, if a vein ruptures, because venous blood is usually dark red, so after the vein bleeds, after a while the venous blood coagulates, a black blood clot may appear. The vascular system also includes arteries, arterial blood is generally bright red, if the arteries rupture and bleeding occurs, after a while, will also form a blood clot, due to the arterial blood is more bright red, so the blood clot is generally not black, but red. Arterial blood is usually bright red because it contains high levels of oxygen, while venous blood is usually dark red because it contains relatively low levels of oxygen. The type of bleeding can be determined initially by judging the color.