The blue blazer is a staple of a man's wardrobe. There is almost no wrong way to wear it, though I would avoid the "boarding school" khaki chinos and blue blazer combination. It is sartorially correct to wear a blue blazer and gray worsted wool dress pants and a tie in place of a suit. You can wear it with jeans and a t-shirt. It was originally conceived as a casual yacht sporting coat for gentlemen and the brass buttons mixed with the navy blue are a throwback to its nautical heritage. That's why Mr. Howe from Gilligan's Island always wore one. One of the best informal-dressy looks with the blazer is to wear it with dress pants and a waistcoat with no tie.