With aristocratic appearance and distinguished manners it is accepted to apply expression to people, unclear even to modern native speakers – "blue blood". What is meant by this idiom, the youth understands only is associative, but also the generation mature will hardly manage to explain accurately.
Aristocrats
All at least once in life heard or used such expression as "blue blood". Sometimes it defines very skeptical relation or allocation from the lump of any given individuals having a set of characteristics which it is obvious or implicit objectively or subjectively force to put them separately among many, or about the persons of a noble family and origin concerning ancient aristocratic family.
Expression does not mean excellent color of the blood cells streaming in veins of these objects at all, however, the history of origin of this familiar expression is connected with structure of blood. Long ago it is known that persons of noble blood, as a rule, were allocated kipelno with white skin color, even easy suntan was destiny of unskilled workers and peasants. It is meant that in veins of similar subjects blood of an unnatural bluish shade flows, as gives them similar, other than usual color.
Expression blue blood has the Spanish and French roots.
Knights
According to other version, medieval knights of especially noble origin did not spill noble substance in knightly tournaments, exactly thanks to its special characteristics and blueness at all. Even inquisition concerned people with similar anomaly with deep respect and fear, saying that similar colors symbolize certain heavenly forces.
Blue blood in the nature
Blue blood meets also in the nature, color of blood is defined than other as its structure. Blue blood often meets at representatives of marine fauna, spiders, Crustacea, this line is caused by special substance haemo cyanine which, unlike human, has a mednosoderzhashchy blue pigment.
Kianetiki - so scientists call people with similar natural anomaly.
The blood cells having a reddish shade in veins of these people I gain blue color thanks to the increased copper content that does not affect ability of blood to oxygen transfer at all. Such individuals, statistically, there are no more than seven thousand people, their blood has a grayish or lilac shade, and this fact is rather an exception, than the rule for this reason expression blue blood - a phrase is rather with a figurative sense, than literal sense.