This holds true most of the time but sometimes it doesn't...meaning there are darker featured people of unmistakable northern Italian lineage and light skinned, or blond, Italians coming from the South (about 20% of my relatives are some dark shade of blond and they are southern).However, that being said, a southern Italian generally would generally get the reaction "now THAT's an Italian." This would be someone like Al Pacino, Tony Danza or Sylvester Stallone. Dark features, dark eyes (especially with what I call the hound dog feature...i.e. the whites exposed under the pupil like Stallone has), a prominent nose and an unmistakable ethnic look. Some are potentially interchangeable with a Greek or a darker Spaniard (they cast Andy Garcia in "Godfather III" and he is Cuban and they cast Al Pacino in "Scarface" and he is Italian).
A northern Italian wouldn't probably get that reaction...they could interchange with French or Swiss people in most cases, which border them to the north. Most of the time, they will have brown or black hair but it's just that they don't look particularly ethnic. If you are ever in Milan or Turin, you will see a lot of Italians that don't hit you over the head as being Italian, particularly the more well-heeled northerners.
Throwing this over to another ethnicity or race, take some Jewish celebrities. I have a Jewish friend I grew up with in LA who was talking about this and she said "now, you take one look at Elliott Gould and you think 'now that's a Jew' "...because he has a certain look. Then you look at Michael Douglas or William Shatner and that wouldn't really register, yet they are Jewish.
This whole anthropological thing is fascinating and people watching in Italy is a trip in itself. Sometimes, I've found myself on trains in Italy trying to keep from busting up in laughter...they probably wouldn't get it and probably don't see themselves as amusing, so they might think I am the one who is nuts.