Black Tie Event - what is acceptable?

I have been to a few of these so can help with my experience.

When it is Mess Dress, you wear very formal, full-length evening gowns.

Black tie is slightly different - these events are usually cocktail/evening dress. So, as long as it's below the knee (that's usually the rules... although not everybody follows it, but they look out of place), any colour, evening style, etc is suitable.

Every mess is different, so check with who invited you to confirm the RSM/CO doesn't have specific requirements.

It's one step down from the full fig, where the men are in Mess Dress and the ladies in long gowns. Just an evening dress is the norm. As already said, you could get whoever invited you to check with the Mess Secretary as to any specific requirements.

Once knew an Irish Regiment, now sadly gone, where for "black tie" events the Mess Members actually wore green dinner suits and the ladies were expected to have something green about their apparel.

Ladies are allowed much more flexibility in these situations than men, and traditionally competed to see who could look more lovely and unique. Wear a long evening gown, long gloves if you want to look more elegant, and some nice jewelry. Do your hair up or at least well-styled. Try to keep your makeup subtle, you want to turn heads, not stomachs :D Think Jackie Kennedy!

Black tie is the equivalent to saying "were a tux". That means it is formal. Think evening dress. Good luck and have fun.