A blue blood is someone from an aristocratic, noble, or old-money family. The phrase implies not just wealth but a long lineage and social status by birth rather than achievement. It is often used in mild irony today, given that most European aristocracies have diminished political power.
The phrase comes from the Spanish 'sangre azul,' used by the oldest Castilian noble families to distinguish themselves from the Moors, Jews, and mixed-heritage populations who had intermarried with the Spanish aristocracy over centuries of conquest and coexistence. The claim was that families of pure Visigoth descent — untouched by darker Mediterranean or North African ancestry — had visibly blue veins visible through their fair skin. It was, in truth, a statement about ethnicity as much as class.
The concept entered English in the 1830s, with early uses in newspapers and novels to describe European aristocracy. By the Victorian era it was established enough to be used satirically. Today it appears most often in descriptions of old-money families, long-established institutions, or traditional thoroughbred animals in equestrian and dog-breeding contexts.
No. Blood is always red — oxygenated blood is bright red, deoxygenated blood is a darker red. The blue appearance of veins through the skin is an optical effect caused by the way skin and tissue absorb and reflect different wavelengths of light. Blue light penetrates less deeply than red, making veins look blue from outside.
In modern usage it appears most often as a historical reference or mild irony. It is also used in sports — 'blue blood' programmes in American college athletics describe schools with long elite traditions.