Ok, a couple of things:
Remember the waves of cultures that have spent time on the Italian peninsula and the surrounding area. In particular to southern Italy and Sicily, the Greeks, the Northern Africans, the Phoenicians all were there in antiquity. As for the 'Nordic' look...remember that Sicily and Naples were 'property' of the Aragonese (Spain)--what is now the provinces of Aragon, Catalunha, Valencia, and a bit of Navarro...all areas where there are plenty of 'gueros' (= blonde hair, blue eye, light skin). If memory serves, there were some silly French k-nig-its (dancing around, knees bent) in the middle of the 2nd millenium AD, but I could be mistaken here.
Oh! and plenty of theories that the Vikings made it to the Mediterranean! (don't know how true this is, though.)
Very notable exception here is San Francisco, whose Italian population is Genovese- (this would be my family) and Toscano-rich, and for the most part didn't see too many Southern Italians until the 1930s. I've heard that Seattle also is very Northern-rich, but I don't know for sure, as I've never been there. But esp. in the eastern part of North America--NYC, Philadelphia, Boston, Toronto, Chicago, to name the big ones--Southern Italians are more the norm.