In my opinion, the biggest difference between formal and black-tie is for the men: formal means a dark suit, and black-tie means a tux. For us ladies, the concept is a bit more nebulous, but I think that you can get away with the same type of ensemble for either dress code. I have a rule of thumb for these types of events: if you wear a long dress, keep it fairly simple. If you wear a shorter dress (which is still okay), then it should be pretty fancy and special.
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