I have been on 11 cruises with Princess, the most recent being November 2007, and jeans are all I wear on nonformal nights. I like black jeans because they look a little dressier, but blue is fine too. I know with the black jeans, you would practically have to be rubbing your hand up and down my pants to tell that they even are jeans. If someone wants to go to that extreme to see what material my pants are made of, then I think they have the problem, not me.
By the say, the Sapphire is my absolute favorite ship. You'll love her!