There really isn't such a thing as a true pure blood anymore. Those that are, are horribly inbreed (look at the Gaunt
family, after all). At least in my opinion.
Then again, it has been mentioned (by fans) that there is no such thing as a real Muggle Born either, that all Muggle Borns are really the descendants of Squibs who married into Muggle families and the magical abilities are just reemerging in a newer generation....but that might be going too far into it and it just plays into my sick idea of an X-Men/Potterverse crossover fanfiction. >.>