I think I may have stumbled across the dress Cordelia wears in the Buffy the Vampire Slayer season 3 episode "The Wish". The dress appears to be by the brand D&G.
I had always suspected it was by that designer brand. The cut definitely fits their style.
The color appears to be a bit darker than the one Charisma Carpenter wears in the episode, but it could be due to room lighting or other circumstances. But the cut and everything else appears to be an exact match! I won't have a better idea, until it arrives.
The dress may also be from a different season, and therefore a different shade of blue. But I do believe that in the episode she is in fact wearing a D&G dress.
I'm happy to finally find this iconic dress. It's one of my all-time favorite Cordelia Chase fashion pieces that screams Queen C.