Prone to Moral Myopia, Blue Bloods often regard only their class as important, which often leads to Aristocrats Are Evil.
The military elite in the Middle Ages and the perceived elite afterwards.
Whether or not they have a sovereign whom they are subordinate to, these characters definitely have commoners who are subordinate to them.
Oddly enough, often characters who have been Made a Slave have former nobility as their Back Story.
The Officer and a Gentleman is also often a Blue Blood, particularly if the code emphasizes the duties and responsibilities that come with noble birth.
Yet another idea on the term's origin, which is erroneous but included here due to the likelihood of the reader encountering it in the context of nobility, is that crustaceans such as lobster literally have blue blood Lobster was neither expensive nor prized until the advent of canning in the 19th Century turned it into a delicacy.
Although this more or less coincides with the advent of the English term, "blue blood", it came some 900 years after the Spanish introduced the concept of Super Trope of Impoverished Patrician, Remittance Man, Noble Fugitive, Aristocrats Are Evil and Officer and a Gentleman.
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In some periods and countries, it can carry the taint of being not quite noble yet if only one's parents were ennobled.
Thus, nobles are often quite proud of the length of their lineage, which makes them the natural foil of the Self-Made Man.
Their position is hereditary, often legally enforced, although occasionally simply socially accepted to the same effect.