Mythbusting time! Colour Lovers settles the "veins are blue" mystery.
Would you believe the answer is pretty similar to why the sky is blue? It's the light, stupid:
Why Are My Blood Veins Blue?
Well, they’re not. They’re a dirty red. Of course, the darker blood won’t be as dark outside of the body because it will oxidize when hitting air, but it still contains waste from cells. Seeing a vein as blue through the skin is because of the layer just below the epidermis and dermis, the two outer layers of skin. Because our skin’s “job” is to protect our bodies from the sun, this provides an interference for how light penetrates the skin by allowing only lower frequency light through, and color is light bounced off of an object, after all. The wavelengths that do make it through to the veins are high-energy blue.
What about the "it's just blue until it hits the air and absorbs oxygen" idea?
Even some of my science teachers have previously said that blood from veins is blue before it hits air… where instantly it’s changed to bright red.
Seen under vacuum conditions, this has been disproved.
Wait, who's cutting people open in vacuums?