I had a black tie wedding in August a few years ago and this would have been very different to what most people wore.
We had a purple midi sequin dress, floor length emerald green embellished dress, red fitted slinky floor length dress, and a few wide leg full length jumpsuits in different materials. All the men were in a tuxedo and bow tie which is the typical UK interpretation of black tie.
I wouldn’t wear anything floral personally, and while I don’t think
floor length is necessary I think it will help you focus on dresses that better fit the theme.