Human blood is always red. It appears blue under the skin because it is viewed through a layer of subcutaneous fat beneath the skin that gives the veins a bluish tint.
The idea that oxygenated blood is red and deoxygenated blood is blue is a common misconception. In fact, human blood is never blue. It does change color depending on the level of oxygenation. In the arteries, where blood is fully oxygenated, it appears bright red, while in the veins, where the oxygen is depleted, it is a dark red.
Blood derives its red color from hemoglobin, a reddish iron-containing protein that is found in red blood cells. Hemoglobin bonds with oxygen molecules in the alveoli of the lungs. The heart pumps oxygenated blood through a branching system of arteries until it reaches the very smallest blood vessels in the body called capillaries. The walls of the capillaries are very thin and allow the exchange of oxygen and other nutrients and waste products between the capillaries and the cells of the body. Oxygen-depleted blood travels through a system of veins until it reaches the heart. The heart pumps the blood back to the lungs, and the whole cycle begins again.
While human blood is never actually blue, certain conditions can give the skin a bluish look. This is called cyanosis, and it is caused by a decrease in blood circulation.