A few blue eyed, blonde light-skinned individuals probably always existed and still do exist in the lands around the Mediterrenean Sea. Which is far from saying that they've ever been the majority. A quick glance to Ancient Greek and Roman art should make you conclude that most people were indeed dark-haired.
There are various accounts that hightlight how the Hellenes thought of themselves as a distinct population. The word barbarian was born in Ancient Greece indeed and was used for whoever was a foreigner, northern european populations included.Here is an extract that hightlights the heterogeneity of Ancient Europe:
Hippocrates (c. 460 – c. 370 BC , Greece)On Airs, Waters and Places.
In Europe too there are tribes differing one from another in stature, in shape and in courage. The differences are due to the same causes as I mentioned above, which I will now describe more clearly. Inhabitants of a region which is mountainous, rugged, high, and watered, where the changes of the seasons exhibit sharp contrasts, are likely to be of big physique, with a nature well adapted for endurance and courage, and such possess not a little wildness and ferocity. The inhabitants of hollow regions, that are meadowy, stifling, with more hot than cool winds, and where the water used is hot, will be neither tall nor well- made, but inclined to be broad, fleshy, and darkhaired ; they themselves are dark rather than fair, less subject to phlegm than to bile. Similar bravery and endurance are not by nature part of their character, but the imposition of law can produce them artificially. Should there be rivers in the land, which drain ofl" from the ground the stagnant water and the rain water, these ' will be healthy and bright. But if there be no rivers, and the water that the people drink be marshy, stagnant, and fenny, the physique of the people must show protruding bellies and eidarged spleens. Such as dwell in a high land that is level, windy, and watered, will be tall in physique and similar to one another, but rather unmanly and tame in character. As to those that dwell on thin, dry, and bare soil, and where the changes of the seasons exhibit sharp contrasts, it is likely that in such country the people will be hard in physique and well-braced, fair rather than dark, stubborn and independent in character and in tempc. For where the changes of the seasons are most frequent and most sharply contrasted, there you will find the greatest diversity in physique, in character, and in constitution.
And here a passage from Aristotle's Politics which shows their perceived separateness from both Asians and Northern Europeans:
The nations in cold regions, particularly Europe, are full of spirit but somewhat deficient in intelligence and craft knowledge. That is precisely why they remain comparatively free, but are apolitical and incapable of ruling their neighbors. Those in Asia, on the other hand, have souls endowed with intelligence and craft knowledge, but they lack spirit. That is precisely why they are ruled and enslaved. The Greek race, however, occupies an intermediate position geographically, and so shares in both sets of characteristics. For it is both spirited and intelligent. That is precisely why it remains free, governed in the best way, and capable, if it chances upon a single constitution, of ruling all the others.
Varg Vikernes was a good musician, but when it comes to historical declarations he's biased by his racist fantasies and I wouldn't ever consider taking him seriously.
This is a theory at least a century old. I ran into it in graduate school, and wrote up what I think about it here. Despite some apparently very modern approaches (e.g. climate change as an engine of history), it turns out to be grounded in deeply racist assumptions. Let's say I am less than convinced.
You should check out the following sections of the FAQ. It will tell you far more than some extremist on Youtube.