This is an interesting hypothesis, but one which suffers from a lack of genetic documentation. If indeed there was an imbalance in the operational sex ratio of early northern and eastern Europeans, then one could discover its signal in patterns of diversity of Y-chromosomal and mitochondrial polymorphisms in depigmented vs. non-depigmented Caucasoids.
My own theory explains the evolution of blondness as a paedomorphic trait that was differentially selected in Caucasoid populations due to its interaction with other phenotypic traits.
Briefly, I propose that hair acts as a frame, drawing attention to an individual's features. The prehistoric northern Caucasoids were broad-faced and coarse-featured (Cro-Magnoid) with a robust type of appearance. Dark hair would act to emphasize these robust features in women, while light hair would act to de-emphasize them, giving a more "feminine" appearance.
After Neolithic times, in the south appeared the narrow-faced and gracile types (Mediterranean), who had a more refined appearance, and these types gradually spread, replacing the coarse types. Coarse-featured women needed an extra boost to compete against the more feminine narrow-faced ones, and light hair provided that boost.
Thus in the long-term there was a trend for selection of narrow-faced women over broad-faced ones, but also a trend for selection of blonde women because of the advantage of robust-blondes over robust-brunettes. This explains the joint survival of blondism and robust types (named Brunn, Faelisch, East Baltic, Borreby, etc.) in northern latitudes.
We can definitely observe the first trend in the palaeoanthropological record, as the skeletally Cro-Magnoid population was gradually replaced by Nordoid types in northern Europe. The second trend cannot be directly observed, but the clinal gradation of hair blondism from a peri-Baltic nexus strongly suggests a more recent spread. This may also be sugggested by some ancient DNA analyses and the possibly late Paleolithic origin of an allele for light pigmentation recently discovered.
Evolution and Human Behavior (in press)
European hair and eye color
A case of frequency-dependent sexual selection?
Human hair and eye color is unusually diverse in northern and eastern Europe. The many alleles involved (at least seven for hair color) and their independent origin over a short span of evolutionary time indicate some kind of selection. Sexual selection is particularly indicated because it is known to favor color traits and color polymorphisms. In addition, hair and eye color is most diverse in what used to be, when first peopled by hunter-gatherers, a unique ecozone of low-latitude continental tundra. This type of environment skews the operational sex ratio (OSR) of hunter-gatherers toward a male shortage in two ways: (1) men have to hunt highly mobile and spatially concentrated herbivores over longer distances, with no alternate food sources in case of failure, the result being more deaths among young men; (2) women have fewer opportunities for food gathering and thus require more male provisioning, the result being less polygyny. These two factors combine to leave more women than men unmated at any one time. Such an OSR imbalance would have increased the pressures of sexual selection on early European women, one possible outcome being an unusual complex of color traits: hair- and eye-color diversity and, possibly, extreme skin depigmentation.