[dekeguy]

Please accept a man's position on this question. Black tie and white tie events are the norm in New Orleans and in London so its almost an everyday thing
An invitation marked Black Tie refers to the man's turn out. You mention that this is a true Black Tie event. In that case I would strongly suggest that your husband wear a black dinner jacket with black cummerbund or vest, a plain white pleated shirt with a turn down collar, and a plain black bow tie, preferably hand tied, not pre tied if he can handle that. The advent of long four-in-hand ties with dinner jackets is theatrical and not acceptable at a "true black tie" event. Wing collar shirts may be worn but really work better with white tie and tails. Trousers should be matching traditional black with leg stripe. Shoes should be black patent lace ups or men's patent evening pumps (yes, they do exist - consult Brooks Brothers on-line catalog). Well polished plain black leather lace ups can be substituted. Plain black dress socks, over the calf work best. Socks should not be clocked or figured. As regards these rules a simple guide would be that the closer you are to the center the less the rules apply, the farther you are from the center the more they apply.
Black tie is by definition "semi-formal". White tie or "Costume de Rigueur" is "Formal". White tie is always black tail coat, white pique vest, white wing collar shirt (plain - no pleats), a white pique bow tie, and white gloves.

Corresponding to the man's attire and depending on the nature of the event ladies attire can be quite different. Long gowns are appropriate for a more structured dance or ball. An up-market cocktail reception would call for your favorite cocktail frock. A structured sit down dinner would call for a dinner frock which could be your cocktail dress or something a bit more restrained but clearly formal. Depending on the event really high end evening pants suits could be appropriate. The range is rather broad so unless you are sure what the other ladies will be wearing I suggest you call the hostess and ask.
As usual its pretty easy for us guys and more complicated for the ladies. According to my sister its better to be a little over dressed than under dressed but long vs. short can be tricky.