When a bit of our body gets bashed, by a fall or an operation, the tiny blood vessels called capillaries break and the blood under our skin forms a bruise. Over time it changes colour as the healing happens.
When these soft tissues are injured, small veins and capillaries (the tiniest blood vessels) under the skin sometimes break. Red blood cells leak out of these blood vessels. These red blood cells that collect under your skin cause that bluish, purplish, reddish, or blackish mark.