Given the Sorting Hat's eccentricities, I doubt that's an actual qualifying blood factor to get into Slytherin House. I'd worry about the qualifications to survive it! It's a House that despises weakness (as they are really excellent at exploiting it). In retrospect I think that's what the Hat was also saying about Harry -- you are brave, but you are also a survivor, and you would do well in this House.

(Hat: Pssst! There's this prophecy thing! You're going to get scragged! Go with Slyth-- mphphphh...!)