I've been puzzling over what blond hair is all about, and there's something there relating to neoteny, or kiddie-fication, withdrawn social orientation, clinging to mommy's skirt rather than fending for oneself, and so on.
The selective forces don't matter yet -- first we have to get a decent feel for how blonds differ from the ancestral dark-haired morphs. Then we can try to come up with stories about why it spread where it has.
One striking clue about blond differences is that they don't appear to appear among the most eminent people in cultural fields. At the very least, they're way under-represented, but so far I haven't found any. All sorts of things keep a person from achieving the highest levels of eminence, but a more neotenous / kiddie mindset and behavioral style would be one of those things.
First, a quick reminder of how common blond hair is around Europe:
So we shouldn't waste time looking at eminent Italians, all of whom are going to be dark-haired. What about where there's a good amount of variation in hair color, like Scandinavia?
I browsed over the Wikipedia article on Scandinavian literature and picked out the ones who even I, a non-specialist, recognize. If hair color is unrelated to accomplishment, blonds ought to make up at least 30%, and more like 50% or more. Instead they are all dark-haired: Ibsen, Strindberg, Knut Hamsun, Hans Christian Andersen, Soren Kierkegaard, Aleksis Kivi (OK, I didn't recognize him, but he's the only Finn listed, and described as one of the greatest in Finnish), and Astrid Lindgren (she looks brown or red-brown, but not blonde).
What about film? Bergman and Lars von Trier are both dark-haired, and so is Paul Verhoeven from uber-blond Holland.
Music? Grieg and Sibelius were dark-haired (even if Grieg is also part Scot), and most pop stars are dark as well.
Painting? Munch is dark-haired. In the Low Countries, so are Rembrandt and Rubens.
Acting is not as creative and does not require as much technical mastery as the other fields listed above. Blonds show up in acting, though even there it may be an under-representation.
That's 15 major figures, and not a single blond, in an area where perhaps half the population has light hair. Yeah, I know, these maps may not have the same threshold for "blond" that I'm assuming -- maybe 80% have "light" hair, but only 10% have yellowish / straw "blond" hair. Still, 0 out of 15. And anyway, just about all of these figures have dark hair, not light brown, so moving the goalposts to include light brown under "light hair" would not change the finding.
And of course, I'm only looking at the figures who are eminent enough for me to already know who they are. I didn't go through every entry on the Scandinavian lit page. But that proves my point -- if there are any blonds to be found, you have to look much farther down the totem pole.
Also, it doesn't seem to relate to "non-dark" hair, but specifically to blond hair. Red hair is also not like the ancestral dark color, but no one ever said anything about redheads being neotenous, kiddie, adorable, cute, etc. -- rather, that they were fiery, temperamental, horny, and so on. Red hair doesn't get up anywhere near 50% even where it's most common, maybe 10-15% max. Still, a quick survey would uncover Ridley Scott as a ruddy-haired, eminent film-maker. And perhaps a couple of the Scandinavians, though more in the auburn band of the spectrum.
Maybe the results would be different if I looked at science, technology, engineering, and math. That's a project for someone else to do. But a quick check doesn't look promising -- Copernicus was dark-haired, as were Niels Bohr and Niels Abel. Tycho Brahe is more of a redhead (both in appearance and behavior -- losing his nose in a duel over a math formula). Linnaeus is harder to check on, though he appears light brown.
A more systematic approach would be to go in order from 1 to however-many in Charles Murray's lists by field in Human Accomplishment, but I didn't bring that with me on my trip.
Whatever blond hair does, maybe that partly explains the relative lack of accomplishment in the Baltic Sea area.