Sometimes a derogatory term for those that are of mixed ancestry. It's all in the tone of voice.
Be careful whenever someone starts talking about the purity of blood. Get ready to run away very quickly unless they are talking about breading animals. For the people that are concerned about purebreds in humans are pretty much universally nuts with massive personal issues. (See also Aryan.)
This can be fun in fiction though. As in half blood fey in stories.