Women wear fancy dresses going to weddings. Saying its a black tie wedding is just to prevent men wearing jeans or overly casual outfits imo. So women would wear whatever theyd wear to any wedding - a nice frock.
Last black tie wedding I was at my OH rebelled and refused to wear a tux because he didnt want to look like a waiter (and like every other guy at the wedding). Instead he wore a beautiful black suit, red fitted shirt and black tie (claiming he was in black tie...lol). Not only did he stand out from the herd for individuality, his outfit was a lot nicer than some of the badly fitting rented tuxedos. The bride and groom both said that the invites said black tie just to be sure the men kept it formal and as my OH was formally dressed, they were more than happy.
The women all wore what theyd wear to any wedding.