Assistant Village Idiot posits "a mutation of light hair on the Baltic coat of Sweden which spread out generations ago." Possibly. But consider. . .
Olof Skötkonung was King of Sweden between 995 and 1022 AD. According to Wikipedia, "In a Viking expedition to Wendland, he had captured Edla, the daughter of a Wendish chieftain, and she gave him the son Emund (who was to become king of Sweden)." See Saga of Hervör and Heithrek at 141. Wendland refers to a region in eastern Germany once home to a Slavic-speaking minority.
Could Edla the Wend and Olof each have had a recessive gene for "light hair"?:
source: Peter Frost, Why Do Europeans Have So Many Hair and Eye Colors? (2006), via Strangemaps
Note: The foregoing is utterly uninformed speculation invented as an excuse to re-post a great map.