In the eighth century, Islamic warriors called Moors invaded and occupied Spain where they ruled for five centuries.
This didn’t sit too well with the aristocrats of Castile, who began referring to themselves as sangre azul (“blue bloods”) to differentiate themselves from the Moors. The Castilians’ blood was no different in color than the Moors, but their skin complexion was lighter than their conquerors.
The Castilian pride in their “blue blood” was a thinly veiled proclamation of pride in their light complexions, and a subtle way of indicating they were not “contaminated by Moorish, Jewish or other foreign admixture”. For the paler the complexion of the skin the more blue the veins appeared.