A black dress at a black tie event is the most classic choice going and it has earned that status completely. The difficulty is that not all black dresses are created equal and the difference between a black dress that works for black tie and one that simply does not is entirely in the fabric and the silhouette. Jersey does not work. Matte cotton does not work. Heavy crepe, satin, velvet, chiffon with weight, silk. These fabrics work. Get the fabric right and a black gown is one of the most elegant and reliable choices you will ever make for a formal occasion.