I agree that Kings and Queens during ancient time, some might have mixed blood. But, because it was at ancient times, that is 1 thing that make it more likely that their colour of skins need to be the same as the general population during that time. There is no royalty during that time who is crazy enough to give someone who is different in colour as them the line of royalty. More than less is that during that time, the person could be killed by assassination's and such. Those years are years of slavery. There is no chance in hell a mixed ethnicity can become the ruler or even the spouse. They might become the mistress but there is not even 1% chance that they can become ruler themself by bloodline. They might, if they win the wars but not by birth, especially if the original ruler was from different breed.
Things during those time was so hectic that some royalty even married their own sibling just to make their blood stay pure. They might have mistress, but for the person who might become the continuation of the bloodline, they usually try not to tarnish the purity of the blood. This is why there is a phrase such as "royal blood".