AHC: at least one of emperor of China have blond hair.
1 ) Did any non-Han people who invade China (Xianbei, Shatuo, Tangut, Khitan, Jurchen) have any member with blonde or red hair ? Do we know what colour Genghis Khan hair is ? (there are some source that he is red-haired)
2 ) Did Chinese Emperor harem had any foreigner inside ? Chinese send princess outside, did they also accept princess ? Is any son of foreigner concubine had any chance to become emperor ?
3 ) how recessive is blonde hair genetically ? can it be received through female-line only ?
4 ) how chinese judge appearance of their prince/emperor ? Did any foreign-looking prince have any chance to become emperor ?
Sun Quan of the Three Kingdoms apparently had purple hair. I'm sure that during the Han Dynasty at least, various Chinese envoys would have come into contact with Greco-Bactrians, Tocharians or other such Indo-European peoples (which is how there are naturally occurring blondes in Mongolia or Western China today).
2) Depends on what you mean by 'foreigner'. Border states would definitely send marriageable women to imperial capitals as 'tribute', and these could range all the way across Central Asia, should the dynasty exert enough power (like during the Tang). Actually, after the Tang Chinese princesses weren't generally married out to 'barbarians', instead being given to close allies/important officials. The Manchus mainly married their princesses within ethnicity, though marriages with Mongol princes were not uncommon.
4) Actually, if the prince/emperor looked strange Chinese subjects would probably see it as a sign of his 'uniqueness' and, therefore, 'divinity' (important for being Son of Heaven). Liu Bei in the
Romance of the Three Kingdoms, for example, is described in weird terms: earlobes so long they touched his shoulders, extraordinarily gangly arms, and 'eyes that could see his ears'. His ancestor, Liu Bang, was said to have a face like a dragon's, and 72 spots on his leg.
That said, I distinctly remember reading (can't remember where) that during one of the earlier contacts with Westerners in the 16th/17th centuries, Chinese people were afraid of blondes both because of the color of both their skin and hair (or maybe because the Portuguese regularly engaged in piracy around Chinese waters). Certainly the 19th C colloquial Cantonese term for Westerners,
gweilo ('ghost people'), doesn't seem like a particularly endearing term, at least not initially.
But if by some genetic weirdness a blonde Emperor
did come to power legitimately, that would be alright - though there'd be more than the usual court gossip regarding whether his
real father was a demon, whether he himself was a demon, etc. etc. And if he was an exceptional (good or bad) leader, his hair color would definitely have been noted for the histories.