Why are aristocrats called blue blood?

Blue Blood is a translation of the Spanish 'sangre azul', attributed to some of the oldest and proudest families of Castile, who claimed never to have intermarried with Moors, Jews, or other races. The expression probably originated in the blue appearance of the veins of people of fair complexion as compared with those of dark skin.

It has been used in English since 1834.

Kings and Queens and other members of royal families are said to have blue blood. But they actually have the same kind as everyone else.

Blue blood is black blood (1100-1848) by egmond codfried and can be read in google. The image of the Moor symbolized blue blood, and the Moor is a classical African. Some European Kings and nobles (1100-1848) were described as brown and black of complexion.