Since there are relatively few Dark/ Ghost Pokemon in Gen III, you don't have that much of a choice.
Main candidates for support would be Umbreon and Sableye; Houndoom is another (offensive) possibility, but since you have Blaziken I would suggest the first two instead.
Umbreon will be able to Baton Pass Mean Look (since this trick still works in Gen III), while Sableye will provide a few useful immunities, though it's lacking in any offensive power. Sableye is not a very good in-game Pokemon unless its bred moves such as Recover.
Offensive Dark Pokemon would be Tyranitar and Crawdaunt. While you have a water type already, I would suggest Crawdaunt. It has less weaknesses that Tyranitar, and you already have Blaziken to resist grass and Manectric to resist Electric.
As for Ghost types, either Gengar or Dusclops. Mismagius doesn't come in until Gen IV, so Gengar for offensive and Dusclops for defensive.