i am not shure but i think it is a slang name for the aristocracy or enny form of royalty. meening that thay are different . but most of all i as said beleve it was used as an inflamitiry remark and name.
If you are talking about the book Blue Bloods by Melissa De La Cruz
They are called Blue Bloods because there blood is blue
Blue blood is black blood (1100-1848) by egmond codfried can be read in Google and explains how brown and black complexioned Europeans became a nobility.
The part above was written by someone else but I wrote the paragraph below.
Well I'm pretty sure they say this because back in the day (I know that doesn't sound very professional or whatever), the wealthy/royal people did not do work in the sun where you would get tan, so they stayed pale and you could see their veins which were blue, so they called them blue bloods, whereas tanned slves, servants, farmers and just workers in general had tanner skin so you could not see their veins easily.
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The Bloods
Blue actually meant romantic or mystical. It certainly isn't blue today but a lot cleaner than it was in the 20th century where it was nothing much more than a sewer.
If you are not in a gang for real, throwing gang signs in the wrong place or in the wrong context could get you killed. No joke. Bloods can spot a bogus wannabe, and they take offense to people who do this.
The same guy who founded the Mexican Mafia which is Luis ''Huero Buff'' Flores is the same guy who founded the Surenos. He gathered a bunch of small Hispanic gangs, aligned them & called them the Surenos cause Sureno in English means Southerner since they were formed in Southern California.
they were the Loyalists. the Patriots wore blue at the time