"Blue blood" along with "white bone" is one of the allegorical designations of nobles, aristocrats. It is hardly worth clarifying that the blood of the representatives of the noble class is no different from the blood of ordinary mortals, nevertheless, the definition exists.
The concept of "blue blood" was born in the Middle Ages. Its appearance is associated with those ideas about female beauty that existed in that era. These views were fundamentally different from those that exist today.
"Blue blood" of the Middle Ages
Modern women of fashion spend time on the beach and even visit solariums to get the coveted "bronze tan". Such a desire would greatly surprise the medieval noble ladies, and the knights too. In those days, snow-white skin was considered the ideal of beauty, so beauties tried to protect their skin from sunburn.
Of course, only noble ladies had such an opportunity. The peasant women were not up to beauty, they worked all day in the field, so a tan was provided for them. This is especially true for countries with hot climates - Spain, France. However, even in England, the climate until the XIV century was warm enough. The presence of sunburn among peasant women made the representatives of the feudal class even more proud of their white skin, because this emphasized their belonging to the ruling class.
Veins appear differently on pale and tanned skin. In a tanned person, they are dark, and in a person with pale skin, they really look blue, as if blue blood flows in them (after all, the people of the Middle Ages did not know anything about the laws of optics). Thus, aristocrats, with their snow-white skin and "blue" blood vessels shining through it, opposed themselves to commoners.
The Spanish nobility had another reason for this opposition. Dark skin, on which the veins cannot appear blue, was the hallmark of the Moors, against whose rule the Spaniards fought for seven centuries. Of course, the Spaniards placed themselves above the Moors, because they were conquerors and infidels. For the Spanish nobleman, it was a matter of pride that none of his ancestors became related to the Moors, did not mix their "blue" blood with the Moorish.
Blue blood exists
And yet, the owners of blue and even dark blue blood exist on planet Earth. Of course, these are not descendants of old noble families. They do not belong to the human race at all. We are talking about molluscs and some classes of arthropods.
The blood of these animals contains a special substance - hemocyanin. It performs the same function as hemoglobin in other animals, including humans - oxygen transfer. Both substances have the same property: they easily combine with oxygen when there is a lot of it, and they easily give it off when there is little oxygen. But the hemoglobin molecule contains iron, which makes the blood red, and the hemocyanin molecule contains copper, which makes the blood blue.
And yet, the ability to saturate with oxygen in hemoglobin is three times higher than that of hemocyanin, so red blood won the "evolutionary race", not blue.
