Human blood is always red. Although it might appear blue through your skin, this is an optical illusion caused by the way light is absorbed and reflected by your tissues.
Blood contains a protein called hemoglobin, which is responsible for carrying oxygen throughout your body. Hemoglobin binds to oxygen, and when oxygenated, it turns a bright red color. When blood loses its oxygen, it becomes a darker shade of red, but it never turns blue.
The misconception that veins appear blue is due to the way light interacts with our skin. Blue wavelengths of light are scattered more readily by our skin, while longer wavelengths like red are absorbed. This results in a blue hue when looking through the skin at the deoxygenated blood in veins.
