I couldnt follow the links.
However, the basques were originally seven tribes, linked more by cultural ties than by blood's. There were the vascones, the autrigones, the eúscaros, and others.
It's not surprising, so, that, although the vizcaya basques we have darker hair, the ones I saw from guipuzcoa have lighter or blonde in a higher proportion, but I don't know so many of the latter.
Anyway, tere have been plent of migrations, like half of the city of Pamplona-Iruña was french population that came through the Camino de Santiago, hence the name "Barrio Francés". Anyway, I think they called French any foreigner who entered the area from France, so it could be any other european origin.
They crossed and settled in Guipúzcoa too, merging with locals and adopting the language. That's why you find surmames with latin (or other) roots, and basque suffixes.
As for Vizcaya, it's difficult to say because we have a 50% inmigration from Castilla, Galicia and Andalucía in this order, so it's difficult to say if we were originally dark haired or not.
The basques origin is not yet identified. So, saying they were indoeuropeans or not, is just speculate. And I don't believe we're darker re skin, never notice any difference with other norhern.
Anyway, about my family, I am not a
pure blood, haha. I have also the northern castilian celtic-iberian mix you probably have.