Eh. Royals in canon ZnT were identifiable by hair color only as long as there were no other crutches for plot; I kid of doubt that author thought hereditary side through, or else magic would not be reserved only for nobles, because 6000 years kind of promote, uh, spreading genes around. For example, Chengiz Khan IRL has now what, several millions of descendants? And it had been less than a thousand years. Now, not everybody is Chengiz, but it probably is safe to assume that most of population carries at least some noble genes. And this is when written history is barely reaching 6000 years (it sort of doesn't, I think..), and medieval-style nobility wasn't in place for most of it.
In ZnT circumstances it means that almost everybody should have some measure of magic, or, in conservative interpretation of nobility's free time habits, chance of being at least weeaaaaakly magical. And the whole "nobility = magic" kind of falls to pieces if you think about it for a couple of minutes.
So yeah, worldbuilding in ZnT is kind of too weak to make logical assumptions; if author wants black hair, hair will be black because it is rather minor offense to logic compared to other poorly thought through bull in setting.