Answer:
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.