Blood is red- blood in the arteries and blood flowing through our veins is dark red. However, when you look at the veins in your hands or legs they appear blue in color.
The explanation for this has been given by Dr. Lothar Lilge and his colleagues. Light is made up of different colors that have different wavelengths which pierce through our skin with difficult effectiveness. The veins you see through your skin appear blue because red light goes through the skin much more easily than blue light.
The red light is absorbed by the blood vessels, and the blue is reflected back. As a result, our eyes see the blue light, and not the red light that has been absorbed. this makes us think that blood under the skin is blue in color.