Narcissa's Hair

So, for the longest time, I've wondered why Narcissa's hair is both black and blonde in the Harry Potter movies (I'm not sure if it was this way, as well, in the books or not). Well, I think I've figured it out. And, if it describes this in the book and I just look like an idiot right now, just ignore me thanks.
So, Narcissa is Bellatrix's sister which would make her a Black (hence the black hair) but she's married into the Malfoy family, known for their white-blond hair and so she takes on their hair color.
I kind of see this as her saying that she belongs as much in that family as her own. I believe that she was rather forced into marrying Lucius and that is one way of her saying that she belongs there. That she wants to be there.
Just my opinion, of course, and by no means correct.
What do you think?