I can't address the biological-anthropological side of the question, but on a purely linguistic note, the word xanthos used of Menelaos does not mean "red-haired"! It means "yellow". It was (infrequently) used to mean "red" in some Roman-era writings, but its earlier meaning always dominated, sometimes with a brown/orange tinge. Ancient colour terms are sometimes problematic because different languages divide up the colour spectrum in different ways, but there's no doubt about this one.

Aristotle refers to the colour between red and green in the rainbow as xanthos; xanthos was associated with the seasons of spring and summer, and not autumn; in Galen, yellow bile is xanthos; the Homeric phrase xanthē Dēmētēr probably refers both to her hair and also to the colour of ripe corn; and xanthos could be used of clouds, the sun, roast pigeon, olive oil, and fried fish. It could also be used of some things that we might think of as "tawny" or "brown", like lions, horses, nuts, honey, and fire; so it doesn't line up exactly with the English category "yellow". But it's definitely not "red".

So the legendary Menelaos was blond. Incidentally, according to Homer, so were Odysseus, Meleagros, Rhadamanthys, and Apollo. Hesiodic poetry makes Ariadne blonde too. Blondness is still not extremely common in modern Greece, but it's not quite as odd as all that.