Why is it said that the nobility has "blue blood"?

The expression "blue blood" is used to refer to people of royalty, nobility or aristocracy. This term has its origin in the Middle Ages, when white skin was associated with a high social status. Origin of this expression There are several theories that tell the origin of this expression. One of them says that during the Middle Ages, the most wealthy people did not work in the sun, so they had the whitest skin. Their veins were visible through pa…