The term "blue blood" is used to describe someone of noble or aristocratic lineage. The phrase originated in Spain, where people with pale skin, a sign of wealth and privilege, were called "sangre azul" (blue blood).
While the term "blue blood" is often used figuratively to indicate high social standing, human blood is never actually blue. The color of blood in humans is determined by the presence of hemoglobin, a protein that binds to oxygen. Oxygenated blood is bright red, while deoxygenated blood is a darker red.
The appearance of veins as blue under the skin is an optical illusion. Veins appear blue because of the way light is scattered and absorbed by the skin and the blood within the veins. The skin absorbs wavelengths of light, except for the blue wavelengths, which are reflected back to our eyes.
The term "blue blood" has no connection to the actual color of blood in humans. It's a historical term that evolved to denote a person's social status rather than a biological characteristic.
