Blood in your veins is blue, and it only turns red when it comes into contact with oxygen in the air.
All blood in your body is red, regardless of whether it's in arteries or veins. Oxygenated blood is bright red, while deoxygenated blood is darker red. Veins may appear blue through skin due to light absorption and scattering, but the blood itself is never blue.
Reception
Sources
- Smithsonian Magazine: Blood Color REFERENCE
- National Geographic: Blood Facts REFERENCE