I don't know that the Hat really considers purity of blood a factor anymore. We've seen three of it's songs; in two, it told us what it (allegedly) considers to define the houses. In the third, it told us who the founders selected to put in their own houses. Particularly since it was open enough to point out that Hufflepuff took "everybody else," I suspect that it doesn't use their selection criteria. Bits of founder brains notwithstanding (GoF).