It's customary for women to wear floor-length formal gowns to black-tie events, but modern style has evolved to offer other options. Since a black-tie invite can signal several levels of required dress, the best way to determine women's dress requirements is to ask the host or hostess.
Black-tie events are always formal, which means men wear tuxedos. Women's choices may depend on the time and type of event. A little black dress is appropriate for an evening cocktail party, and a darkly hued cocktail dress looks more formal than a bright dress, says StyleCaster. Color is less important with long dresses, which are usually considered formal, and women may also couple a full formal skirt with a blouse.