The English idiom Blue Blood has been in use since 1834 and refers to a person of noble birth or descent. It is a translation of the Spanish phrase sangre azul, which described the Spanish royal family and other high nobility. Thought to have originated from ancient and medieval European societies, the term was given to distinguish the upper class whose veins appeared blue through their untanned skin. In contrast, the working class (mainly agricultural peasants), had tanned skin from working outdoors making their veins less visible.
An alternate explanation for the origin of the term is the high incidence of hemophilia among royal families. Hemophilia been called “the Royal Disease”,and has its origins in the family tree of Queen Victoria. The disorder spread throughout Europe’s monarchies and those with the disease, who were over protected, did not go outdoors often and their skin appeared bluish. (Wiki)
