Black and blue bruises from a fall on her knee cap
A bruise, also known as a contusion, is a type of hematoma of tissue, the most common cause being capillaries damaged by trauma, causing localized bleeding that extravasates into the surrounding interstitial tissues. Most bruises are not very deep under the skin so that the bleeding causes a visible discoloration.
Bruises usually fade away in about 2 weeks. Over that time, the bruise changes color as the body breaks down and reabsorbs the blood. The color of the bruise can give you an idea how old it is: When you first get a bruise, it's kind of reddish as the blood appears under the skin.