Eh, I'd say that's generalizing a bit, Serana is proof that Vampires have the ability to be pretty amicable. Hell she will join you and pretty much destroy everything about the home she grew up in if you join the Dawnguard. Id imagine that it would take an extraordinarily strong willed person to overcome the dominating nature of a Vampire but I don't think it's at all impossible
This as well
EDIT: What exactly is your question about them? To be honest beyond what is said in Dawnguard there isn't much documentation on Purebloods other than what's written about Lamae Beolfag. Keep in mind that if you are playing a Daughter of Coldharber I'd imagine that ritual is something that never truly leaves you (I play Rose as still having vivid nightmares of it, one of the reasons she chooses to rarely sleep) and would need a very strong purpose and connections in order to stay focused on not becoming a blood thirsty monster.
Sorry to keep adding XD, I'd say try and flesh out how she became a pureblood, was she a volunteer? Was she forced? what was she before? after? how did she cope? If she volunteered, what made her turn towards good? How old is she? so on and so forth.