When/How often to use Dark Arts?

This thing seems like a serious MP consumer to use constantly.

Dark Arts should be used whenever you have an excess of mana during an encounter, a general guide I use is keep above 50% mp for each trash pull and dip no lower than 30% while fighting to give me room for a quick unleash if needed. DA Souleater is a huge dps gain even when main tanking and DA Dark Passenger's blind effect is noticeable on 3+ mob packs.

Thank you :) that is extremely helpful. I was feeling pretty lost about it.

With excess mana whenever you can.
Single target your excess priority will be Carve and Spit with Dark Arts > Dark Passenger with no dark arts > Souleater with Dark Arts

Though the two first are so good that they shouldn't be considered an excess and rather just be used as a priority. You'll never want to use Dark Passenger with Dark Arts on single target bosses as you'll have a finite amount of mana and its less potency than using it on a Souleater.

If you need some extra mitigation you can also use it on Dark Mind and Dark Dance, though thats more of a situational judge it yourself thing.
On AoE use it for Dark Passenger if you have blood price up at the moment, aside from that you want to use it on Dark Passenger if you feel like you want the extra miss chance on the mobs, similarly use it on Dark Mind or Dark Dance if you feel you need more mitigation. For damage purposes when not in infinite mana blood price mode you'll get more potency for your mana just not using Dark Arts at all on AoE. Similarly if you are a bit behind on gear you might want to open with a Dark Arts Power Slash combo just to give yourself some leeway with the DPS. This is especially noticeable if you're fresh 60 and have people fully tome geared DPSing.

As long as I don't fall below 20% of MP I use it, on what to use it depends on the situation.

I use it whenever I am fighting a mob/boss that does not require me to use Unleash. Speaking of Dark Arts, anyone else know what level we get the animation that shows black flames on our feets? I have tried looking at videos online and the only Dark Knight that has this skill is the one in the HW Job preview trailer. Is this skill still in the game?

Thanks guys. I appreciate all of the help. :) This is kind of hard to wrap my head around by just reading. Going to take lots and lots of practice.