text: Blood without oxygen in it is dark blue.  It turns bright red when it contacts oxygen, either by leaving the body through a cut or by recieving oxygen from the lungs.  If you cut a vein (which transports deoxygenated blood to the heart and lungs for refueling) the blood will come out almost black for an instant before it hits the air.\n\nIf you look at your veins in places where they're easily visible, like your wrist, you will see that they appear blue, not red.