[melvinmt]

You're absolutely right, it probably does originate from the colonial/medieval days but nowadays it's just an expression, at least in Western Europe.

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.

[lotsofmangos]

One thing is that there is still strong usage of terms like blood-line, half-blood and blue-blood in English within fantasy novels and period dramas, however the reason they are popular in those settings is precisely because they feel so archaic.