Very well, as you wish. It is much more likely and faster to get already existing mutation from species who you can breed with, than develop spontaneous mutation. For spontaneous mutation you can wait from one generation to infinity. It doesn't really need to happen at all. You have to keep in mind that population way back was much smaller than today, therefore probability of this mutation was also much rarer. This is just a question of probabilities, and likelihood.
You just speculating, it's not a proof. It is possible that blond hair in their tribe was more common in the past, and now they are "regressing" to black hair. Who knows maybe in few generation their blond hair will be very rare or extinct. There is no need for blond hair in this climate, therefore this trait might be recessive. Anyway it exist only in kids. They might have just some melanin producing gene turned off till puberty. The mutant founder effect?
IIRC the Iceman from Alps had twice as much of Neanderthal genes than today's europeans. It means we are loosing original Neanderthal genes. Well, I guess genes mutated, and I think we have more Neanderthal's DNA but it is becoming unrecognizable after many mutations.
During Ice age climatic zones were pushed down south. It is logical then to see white Neanderthals around mediterranean.
There are few genes involved in it, not one, so it is obvious that Northern Europeans developed extra mutations for extra light skin. I'm just saying that Neandertals gave them a "kick start". They made possible for Africans to move North into Europe and Asia. Otherwise black Africans would have been dead in few generations in Moderate climates.