text: color of blood? i have one teacher that says that blood is red and my other teacher says its blue or green( i dont know exactly) it only turns red when it has contact  with oxygen. so what is it? 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.  if you look at your veins in places where they're easily visible, like your wrist, you will see that they appear blue, not red.