well like u said that small detail was just entirely to confuse the reader and even if not then honestly it could be boiled down to the fact that ppls hair can get darker with age
Wait, for real?
Aham, when i was really young my hair was blonde but as I grew it turned brown
I'm sure the author probably didn't intend it but there's actually a lot of babies born with blonde hair and blue eyes that darken over time. Genes are wild, they might suddenly turned on or off as you age so many blondes become brunettes while their once blue eyes might shift to green or brown also.
It is concerning the flashback with the prince and the girl
The flashback shows that the girl had light hair coloring (this is due to the fact the dude didn't see her hair so he just left it blank in his head) and this was used as a misdirection for us the readers to think that it is MC
The misdirection only works because it is a manga with only black and white so we could not tell that her hair actually did not have a color and was just left blank, and it was not blonde