Why have all Uchiha's dark hair? It's just genetics and Kishimoto-sensei who decided that.
The reason why Naruto is blond, is, like someone already answered, because Minato's blond hair genes were apparently more dominant than Kushina's red hair genes. *shrugs*
So if Naruto and lets say Hinato for example were to have a child, would that child possess the byakugan. I mean as far as I know, all characters with sharingan or byakugan have parents who are both from the same clan.
This is one reason why I don't think Naruto and Hinata will end up together. I mean it would just be weird, Kishi would have to troll us for it to make sense.