All right. It's definitely well written, that's clear. And it's got a lot of potential but there are a few things that I have a problem with. First, you probably already know but JK Rowling did clear up the whole Blaise issue. Blaise is a boy, it's come from the author herself and I guess I just have a little problem with some one changing something as big as gender to fit their story.
Second, the whole "she was put in gryffindor but went to slytherin instead". Considering the sorting hat yells out the name of the house they're put in I doubt in a very large way that Dumbledore would allow her to go to Slytherin just because her parents are obnoxious.
Third: I don't know if you're planning to explain why Dumbledore took her to Harry's house but I doubt there's anyway you can make it seem reasonable. It just doesn't make sense. Not to mention that you have Blaise upset about how she was forced to treat harry badly and yet he welcomes her with open arms. In canon, Harry despises anything slytherin and isn't very likely to willing (and happily) let a slytherin stay with him.
Fourth (and you may have seen this coming) your Blaise has crossed that oh so dangerous line into Mary Sue land. She's oh so beautiful, not scared to challenge her parents to do whats right and judging by the turn this has taken, is going to end up with Harry as his One True Love, right?
I really do apologize if any of this comes out as too harsh. I'm honestly trying to be helpful because a few descrepancies can really ruin a story.