You can't really say that Dark Knight has "always been a DPS" because prior to XI and XIV, jobs weren't classified that way. A majority of jobs in FF history have been damage-dealing with some sort of specialty. XIV needs to be flexible on adapting well-known jobs in order to include them, otherwise everything will be a DPS.

As for their signature move, Darkness, that can still be adapted as an AoE attack with massive hate. And there are still ways for them to sacrifice health to gain other buffs or to debuff the boss. Dark Knight can be a tank while holding to true to its style.