Correct me if I am wrong, but I really don't think you could get a "Dark Lugia" even if you wanted to.
The only thing resembling a Dark Lugia is Shadow Lugia, from XD. and to get that into Gen IV, you'd have to transfer that to a third generation game. This instantly poses a problem: XD will not allow you to transfer unpurified (that is, Shadow) Pokémon. However, even if you could transfer an unpurified Pokémon, neither Gen III or Gen IV have the necessary sprites to display it, so your Lugia would be the normal colour.
So, the chances of this trade ever happening is, I'm afraid, absolutely nil.