Hi Glamgal,
To answer your question simply, either will suit! But I can understand the need for clarification as recieving an invite to a wedding with a dress code requirement can often cause confusion and a wave of panic among many. But rest assured as the term 'Black Tie' translates into Formal Evening Wear. Think of a Social Ball or a Dinner Dance? No confusion on the dress code there, so equally a Black Tie Weddings are of the same ilk.
There is absoluley nothing wrong with asking the bride or groom to clarify the terms of their dress code but in the meantime, I hope this helps. Modern eqitette now allows for both cocktail dresses and floor lenght gowns, just once you remember your classic LBD probably won't cut it. Rather keep in mind that you and the other guests are dressing to impress. You can be assured that this wedding will be a glamourous and formal affair, so don't be shy about blinging up your look.
Also, its always a good idea not to outshine the bride and also considerate (in a black tie senario) to ask what colour the bridesmaids will be wearing to avoid both mix ups and clashes. So its up to you whether you would prefer knee or floor lenght, just once it wouldn't look out of place on the famous red carpet! A word of advice, beware the popular high street brands of evening wear, which are mass produced each season....you don't want your day ruined by someone else in your dress. In cases like this you are safer with boutiques and bridal stores that stock evening wear. And not forgetting, in this time and age there is no need to fork out(unless you want to that is!) on a new evening dress when you can hire one, or buy secondhand, no one need know. Renaissance (042 9335535) in Dundalk have a lovely collection of new and second hand gowns, while Glitz (086 3492178) offers a hire service, to name a few.
I've added a few links for ideas below...best of luck and most importantly, ENJOY!!
http://www.karenmillen.com/fcp/product/ ... 3000053097http://www.flirtprom.com/dress.aspx?page=3&style=P2316http://www.designerdesirables.com/desig ... e-9866.htm