I think the obvious answer to this question is no, that noble blood has no inherent "better" properties to separate it from peasant blood. That the only really difference is the way different wealth classes are raised and live day to day - that if anything, constant "noble" breeding among a select population had instead lead to matchups within one's own family line, leading to weaker genes if anything.

But there is just such ubiquitous fixation on discriminating for "pure" and "noble" bloodlines throughout history over cultures across the world (still seen even today) that I can't help but second guess myself if there really may have been some special property to noble blood. Mayhaps in the past? Before bloodlines became increasingly mixed over time?

So is there answer to whether or not noble genes/royal blood ever had a difference that legitimized its separation it from common blood?

To expand on this question, I've noticed that in many instances in history, there may often be "take-overs" where one noble ruling power becomes unseated. And what ends up happening is that the take-overs, be they from significant lineage or not, will suddenly be the new "noble" power. I think occurrences like these demonstrate that noble blood has no real difference from common blood (as they can interchange, ie a noble celt would be a common roman), but more revealingly, bring up the question of: why was noble blood such a commonly used claim to legitimacy?