Dark Knight is different in every title.

From the weapon it uses to its skills, abilities, lore and job role (and let's be honest, excluding FFXI, every job (except healers) in every game is an auto-attack DPS anyway, including Paladin). So let's stop arguing whether DRK is a tank or a dps, because in this game it has been decided it is a tank, and in this game, that is all that matters.