"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:
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.