Yea, my understanding is that the 'unification' mess dress was 'midnight blue'. Later, when the army and navy folks transitioned to their traditional colours for mess dress, the air force found just inherited the midnight blue option as their own (close enough, eh?).
With the return of the traditional blue option, many RCAF members continue to wear the midnight blue option dress to this day ('grandfathered'). Personally, I like the midnight blue option as it's so dark, it almost looks like a black tux in low light.
Photo attached of midnight blue mess dress with army insignia.