Why do bruises appear blue?

 When one is hit, the force of impact damages the capillaries or the small blood vessels under the skin although the skin may remain unbroken. When a capillary breaks under the skin, blood oozes out and this internal bleeding gives skin its dark colour. Immediately after the injury the colour is red because of the presence of red blood cells. Gradually these cells dry and start losing oxygen and give the bruise a bluish colour. The thickness of the skin also distorts colour. A bruise may change from reddish blue to greenish blue and then yellow before fading completely.