Hmm. Interesting. I do love the white and blue dress. Anyways... I have to admit, being Edwardian, I have trouble imagining black tie at 3 in the afternoon. Black-tie by those rules is strictly after 6 p.m. Before then, we would be talking black coat (no tails) and grey (i.e., striped or checked) trousers. But since we're talking a wedding... I've never thought about this before... I wonder if this isn't too casual...
O.M.G. A BIG *F* FOR ME. I HAVE FAILED AN ETIQUETTE TEST. I'M SO ASHAMED. SOMEONE'S PARENTS HAVE SOMETHING TO ANSWER FOR.
...Whoops. Ok. Sorry. I'm back Where was i? So ok, of course, what black tie means, really, is "semi-formal." However, a hundred years have passed. For us people of today, semi formal is as formal as it gets. And all that stuff about ties is for men. So gowns.
... And I see you've asked - and full length gowns it is. The next thing would be how much spangle would be appreciated (which would be a cultural thing). And also because we're talking Coptic church wedding, I'd think you'd really need to be covered in church. Possibly to wrists and neck, and also head...