Katty brings up a good point... Perhaps in his younger years he was an outcast, but as he became famous everyone began fawning over him. This would certainly be enough to make Krum even surlier and more withdrawn; after all, I'm sure it can't be pleasant knowing that people only like you because you're good at Quidditch.

I like the idea of the Muggle mother, actually; I don't think that would be at all difficult to believe, and like SiriuslyMental (sorry, I don't know your name ) said, there wouldn't be enough Purebloods to populate the school. In fact, I'm sure that Purebloods would be a minority; I don't think he would be overly discriminated for being a Half-Blood (at Hogwarts, it's only Muggle-borns who are picked on--not Half-Bloods).