You got the question wrong, actually, it's usually americans who think the only whites in Europe are the pale, blond and blue eyed scandinavian, which is, of course, stupid.
A darker tone of skin doesn't necessarily means mixing with africans, that's incredibly ignorant.
The most important factor in the darker tone of skin of southern europeans is simply, EXPOSURE TO THE SUN, due to the lovely weather there is in southern europe...
The portuguese, for example, which are, for the most part, brunette, brown or black hair and brown eyes are the LEAST mixed people in Europe and the ones with the oldest genes, most portuguese are direct descendants from the pre-historic peoples that have always inhabited what is now Portugal.
Ignorant people refer to the arabs invading southern europe but don't know that the Moors type of colonization wasn't based on "peopling", that is, in the countries the Moors invaded only a small military and political elite was in fact arab, ruling over the already existing population.
The same can be said of the Romans, who before the Moors were a much stronger presence in nearly all european countries and their genetic impact was so small it can't be accounted for.
Finally, those ignorant fools who think the only whites are northern europeans forget that Modern Civilization, Art, Philosophy, Mathematics, Physics, Medicine, Politics and Social Organization were created by the very dark skinned GREEKS in a time when those pale blondies from the north were nothing but dumb barbarians who could barely read or write.