The reason for the blue colour is that the blood returing to the heart is darker, because it is rich in carbon dioxide. Blood is delivered to the veins from capillaries after the exchange of oxygen and carbon dioxide has taken place. Veins transport waste rich blood back to the heart and lungs. Valves located in the veins insure that blood keeps moving in the right direction, towards the heart and not be allowed to flow backward. The waste rich blood flows back the to the heart, and so the story starts again.
Oxygenated blood is bright red in colour.
Waste-rich blood is dark red in colour. The average person has about 5 to 6 liters of blood in their circulatory system.