Blood is always red, even without oxygen. The reason why veins appear blue is due to the way that skin and veins absorb and reflect different wavelengths of light. Veins themselves are mostly colorless, and it is the blood in the veins that gives them color. Oxygen-rich blood is bright red as it leaves the heart, and when it returns to veins with less oxygen, it is still red but appears darker, so veins look blue. Therefore, blood is never blue, and the idea that deoxygenated blood is blue is a myth.