I'm not an expert in genetics, either, but I remember learning that the recessive gene is carried. My parents both have dark hair and dark eyes, yet I was a blue-eyed blond (which changed to darker hair later). Both parents have to share the recessive blue-eyed gene, for example, and the child must inherit both recessive genes to have blue eyes. (I guess it works the same way with hair color). That's why my children all have black eyes, even though mine are blue, because their father doesn't carry the recessive blue-eyed gene. But all my children WILL carry the recessive gene. So, if they marry someone with blue eyes or a blue-eyed gene, they could have blue-eyed children or at least children who pass the recessive gene along... (Hope that made sense).
That's why two-hundred years sounds like too short a time for natural blond hair to disappear from the face of the earth. But, hey, better claim minority status now before someone catches on to this study! ;-)