But it wasn't about inherent biological differences between people at that point. The descendants of a peasant living by the castle of the first king would be biologically similar to the descendants of the first king. The jews were biologically similar, to the point where a particularly insane (the disease being religion...) king was opening his grandmother's tomb to check whether she had a tail and must have been jewish. This was a few decades after the expulsion of the jews in the early 16th century. Jews would be recognized only because they had hidden tails and the king might have "unclean blood"! It was religious madness, not something due to biological differences. As the nobility closed ranks to monopolize power and privilege for their own descendants, this "blood purity" test just happened to be easier to do when there were obvious giveaways such as skin color: no descendant of the old nobles could possibly be black. Kings might make a commoner noble despite this closing of ranks by alleging that some ancestor was noble, or even that obviously valor proved such-and-such was of unacknowledged noble birth (illegitimacy was common...). But no such finessing could be done of the person was black! It was the nobility against parvenus... a phenomenon common even among the upper castes or any country today.