The biological correctness of it didn't even cross my mind; blue blood, for example, is the term we use when we talk about royal ancestry. It's not like we think that the blood of the royal family is actually blue, however the term does originate from a time where royals were lighter skinned than the working class of peasants, and thus their veins appeared to be more blue than others.
Just found out that mixed blood means mixed European and Native American ancestry specifically in the US (http://en.wikipedia.org/wiki/Mixed-blood).
I don't know if it's racist but it's definitely racial :)
Bloodlines sound more like something from royal families and the aristocracy, of which we still have a select few in NL. Baronesses and whatnot. Kinda posh.