"Blue blood" is an English/American idiom that refers to high-class and/or rich people, with the implication of effete arrogance.

Two false theories have been presented to me at various times to explain this:

  • Wealthy people tend to use silver cutlery, and because of all the extra silver that gets into their diet from this, it gives their skin a gray or bluish cast. Now, it is true that too much silver will discolor your skin, but using silverware alone is not enough to do that.
  • Blood inside your body is actually blue, but the reason you never see blue blood come out is that it oxidizes on contact with air and becomes red. This just isn't true. The redness of blood does change based on how much oxygen it is carrying, but it only varies from bright red to dark red.
The term is actually derived directly from the Spanish sangre azul which was used by noble families to indicate pure breeding. Their lighter skin showed its blue veins more clearly than darker skinned peoples like the Moors.

But, you may ask, how come my veins look blue when they are actually clear and my blood is red ?

That question was answered by Lothar Lilge, a physicist at the Ontario Laser and Light Wave Research Center in Toronto. After a series of experiments he discovered that the outer layers of skin reflect diverse wavelengths of light differently. Red light penetrates deeper into out skin than blue light, so the red light makes it into the vein where it is absorbed by the blood. But the blue light gets reflected off the skin above the vein thus giving it a bluish coloring. When no vein is present the red light does not get absorbed and reflects along with the blue.