Can you wear a dark blue shirt and black slacks to a black-tie optional wedding reception?

No, you may not wear a dark blue shirt and black slacks to a black-tie optional event. Even though the black tie is "optional", it still indicates that guests should wear very formal clothing. It is likely that many of the guests will be wearing tuxedos, but a dark colored suit preferably black) with a crisp white shirt and a darker tie are still appropriate. At the bare minimum, you might be able to blend in without the jacket, but it is best to wear the full suit. Ultra formal events, which are rare in America, are "white tie formal". This includes things like the Inaugural ball for the President. Just below this is "black tie formal", which includes things like the Oscars. "Black tie optional" is just below this distinction, so these events are still considered very formal. Hence, the need for the dark suit if one cannot wear the tuxedo. For more information see this wiki post: http://wiki.answers.com/Q/What_does_Black_attire_mean