Translated from the old Spanish phrase "
sangre azul", blue blood derives from the Medieval belief in Europe (among other places) that the blood of the royalty and nobility was blue; since the royal family and aristocrats were wealthy and powerful enough to pay
commoners to labor in the fields for them, their skin was translucent and pale enough for their blue veins to stand out.
It also refers to old money families: families that have been aristocrats for many generations.
The blue blood disdainfully looked upon the unrefined manners of the
nouveau riche (aka "
new money").
The blue blood of the elite could not be tainted by the blood of
commoners, lest the whole line be polluted (disregarding the risks of inbreeding).