The phrase "black tie" typically refers to a formal dress code for social events. It means you should wear a black bowtie and a black tuxedo, while women should wear an elegant evening dress. Full definition
It is such a gorgeous color and midi dresses are definitely my go to for black tie events. (mackenziehoran.com)
Although evening gowns are standard fare for black tie weddings, you can get away with a formal cocktail dress (of the longer variety) or even an elegant suit. (brides.com)