I happened to ask myself the same question yesterday, ran sims on PokeBattler, and discovered that Arc-double dark does about exactly as well as an Arc-double fire, that is, not terrible, maybe good enough for a B team, but not A team material.
If you are up against Mewtwo with Focus Blast, it becomes definitely good enough for a B team, doing just barely less damage than a Tyranitar B/SE and fainting just barely sooner than it, too.