When you are beated badly, why does yr skin turn blue-black on that area?

The blue and black areas of skin are from punctured or busted blood vessels even veins under the skin.

Explanation: An abrassion even blunt force, may often times burst the vessels and veins under the skin without breaking the skin on top. If the skin were lacerated; it would bleed profusely. The color you're seeing is the color of your blood before it is mixed with oxygen turning it red. Therefore the dark blue and purple color is in fact blood.

When you get a bruise or a other injury, your skin can turn black and blue. This happens because very small blood vessels which are called capillaries broke. Capillaries are the smallest of all your blood vessels, only visible under a microscope. When the capillaries break, this can cause bleeding inside your body which is called "internal bleeding."