Yet, all depends on what somebody call 'black' hair? (+ the question of head hair as opposed to all body hair and beard and...Look at today "Celts" with often head hair darker than the other parts of body)!
Central Europe and western Balkans have relatively rare 'jet black' head hair compared to other dark head hair put into the bag of "dark" in the most of German ancient works, but which are rather blackish brown or very very dark brown, and are common in western Balkans (45 to 55%, until 66% in some small part of Serbia). When I say "are", do translate "were" in the 1950's and a bit later...
As I wrote before, jet black head hair is very very rare in Scandinavia as a whole when speaking of true Scandiavians, not immigrees, it would be around 0,2% (Sweden) or very close to. The broader category of 'dark' head hair were between 3,5% and 5% according to regions, left aside Norway Nordland, Troms & Finnmark and other regions where Saami's weight is important. The maxima of 5% were observed in Western Norway : Hordaland & Rogaland, regions of Bergen & Stavanger, and in Malmö region, in southern Sweden. Ancient maps showed a similar case on a narrow strip of land on the shores of Upland in the northeast of stockholm where a slave trade was active in Vikings times, if I don't mistake. In Denmark, the North of the Sjaeland, island of Copenhagen was a bit darker than the country mean, and the Samsö island East of Jylland (Jutland) was considered as the darkest, but I have not estimation. The Färö Islands are the darkest, I think, maybe around 7-8% dark head hair.
3,5% and 5,0% seems little thing, but in fact it corresponds to an increase from ~30% to ~45% of diverse brown hues (slightly dark to light brown) and the corresponding decrease in light head hair!
For Iceland, the 4,5% of dark head hair could be correct too. But I have no idea of the true % of 'jet' black hairs there.
Today surveys about head hair colour are very vague, the scientists rely more on bones metrics.