The term “blue blood” means a person of noble birth. It is an idiomatic expression that originated in Spain, referring to the paleness of aristocrats who had never married into Moorish or Jewish families.
In medieval Europe the nobility did not work for a living, and so remained untanned by the sun and wind of the fields. Their veins showed blue through their translucent pale skin. The Spanish royal family was described with the phrase sangre azul, directly translated as "blue blood." They described themselves thus to show that their lineage was not intermingled with the Moors, who had a stronghold on the country at the time.
Hemophilia, a disease that causes excessive bleeding, is a familial tendency among some European aristocrats and another possible origin of the phrase. Those suffering from the disease were sheltered and had the characteristic pale skin. Hemophilia sometimes causes a bluish cast to the skin when blood is low in oxygen, literally making the blood look blue. Queen Victoria’s sons and members of the Russian Romanovs royal family suffered from the disease.
Around the 1830s, the phrase began to show up in the English language. Novelist Anthony Trollope used it in The Duke’s Children, the 1880 finale to his Palliser series. This is evidence that "blue blood" was already part of the English vernacular at that time.
An idiomatic expression does not mean what the individual words mean. In the case of blue blood, blood is not literally blue, but only appears so through the skin. Idioms can be confusing to people who are unfamiliar with them. Blue blood is a transparent idiom, meaning that the definition is inherent in the phrase itself. This means that a listener unfamiliar with the term may be able to infer the meaning from its context.
As of 2011, the most visible blue blood the English aristocracy. Spain, the Netherlands, and Monaco also have royal families that are still quite prominent in the press. Royal weddings in particular tend to captivate the masses in newscasts and Internet searches.