It also refers to old money families: families that have been aristocrats for many generations.
The blue blood of the elite could not be tainted by the blood of commoners, lest the whole line be polluted (disregarding the risks of inbreeding).
Now it generally refers to people from old money: families who have been wealthy aristocrats for generations on generations.
Henry lives in Greenwich, Connecticut; summers in East Hampton, goes to Phillips Exeter Academy and plans to attend Yale in the fall. He's a blue blood.