Black tie is one of those occasions where the dress code sounds completely clear until you are actually standing in a changing room trying to work out what it means in practice. Floor length is the obvious answer but not the only one. The real requirement is that your outfit looks like it belongs in a room with chandeliers and people who made an effort. That rules out a lot of things but it also opens up considerably more options than you might think. Black tie does not have to mean a plain column gown in a safe colour. It just has to mean properly dressed. There is a significant difference between those two things and it is worth knowing.