Big props to
@AntG168 for the humility in the exchange.
More teaching:
Blue blood is likely a racist term translated from the Spanish "sangre azul". A sect of the Spanish nobility used the term to describe that their skin was pale white and thus it was easy to see the veins of blue blood underneath. This implied they did not have any breeding in their past lineage with the dark skinned Northern African Moors who ruled Spain for a long period. It also implied they came from money because their pale skin showed they did not have to work in the sun. The Moor rule of Spain was 700 years long, so to keep a noble bloodline both unbroken and without Moorish intermarriage for that full period was a source of great pride for those that coined the term.
It was adopted by other European nobility and eventually Americans who downplayed the racism aspect of it and emphasized the long lineage & wealth aspects, which is why colloquially it's used to describe a person or institution of old money or tradition.
It's basically the opposite of mudblood from Harry Potter.