Not all Scandinavian populations had blonde hair. I know a blonde person of unmistakable Scandinavian extraction who, it turns out, possesses a Y chromosome hap

Not all Scandinavian populations had blonde hair. I know a blonde person of unmistakable Scandinavian extraction who, it turns out, possesses a Y chromosome haplogroup from Northern China, presumably inherited via the Sami or a similar herding society that moved across Siberia. See https://en.wikipedia.org/wiki/S%C3%A1mi_people https://en.wikipedia.org/wiki/S%C3%A1mi_people AFAIU, because of how the inheritance traits for hair and eye color work, a population could have a relatively persistent fraction of people exhibiting non-typical phenotypes for some time even with a temporary infusion of alleles, not to mention regular contact. In general, people got around more than one might think. The Inuit reached Greenland shortly after the Vikings from the opposite direction; both ended up displacing the existing inhabitants. The Age of the Vikings is pretty much modern times in terms of speed of migration and population mixing; not much different from today as compared to when many stereotypical phenotypes originally arose.