Hp-lexicon, наиболее точный источник из известных мне, в этом смысле однозначен:
"blood traitor - A pure-blood who doesn't insist on maintaining his or her purity.
pure-blood families tend to be exclusive, looking down on those who are half-blood or Muggle-born. A pure-blood who doesn't hold these kind of prejudiced views is called a blood traitor".