What's the meaning of the phrase 'Blue blood'?
The blood that which flows in the veins of old and aristocratic families.
What's the origin of the phrase 'Blue blood'?
'Blue blood' is a literal translation of the Spanish 'sangre azul'. This was the designation attributed to some of the oldest and proudest families of Castile, who claimed never to have intermarried with Moors, Jews, or other races. The expression probably originated in the blueness of the veins of people of fair complexion as compared with those of dark skin.
That was certainly the understanding in the 19th century, and there seems little reason to doubt it. In 1834 the Anglo-Irish children's writer Maria Edgeworth published a novel entitled Helen, which reiterates that opinion:
"[Someone] from Spain, of high rank and birth, of the sangre azul, the blue blood."
These days there are fewer people of noble birth who make a point of maintaining a fair complexion in order to display their rank. We are more likely to see apparent blue blood in the extremities of those suffering from the poor circulation condition Raynaud's.