A black-tie dress code is formal and requires you to wear classic and sophisticated clothing. For women, this is an evening gown or a floor-length dress. You do not want to wear anything short, revealing, or bright and bold colors.
You can certainly wear a tuxedo, but for those who don't, go for a suit in a dark color such as navy blue, black, or grey. "A black-tie optional wedding is no place for your favorite black or red button-up—save this for a guys' night out!" And don't even dream about skipping a tie.