Basically, only use your power slash combo to generate initial hate on a boss or pull extra aggro if your group out-gears you significantly. Otherwise, you will be using dark arts on the souleater combo when you have mp or using delirium when you don't (which you will get soon). The idea of DRK is to balance your mp drain to maximize your damage/mitigation. You should never have full mp, in fact you should generally try to hover around the 25-50% range of mp in fights.

Use dark arts on souleater whenever you have the mp to spare, otherwise spam delirium once you get it (power slash for now). Blood price should be giving you enough mp to use dark arts and/or unleash fairly often, especially in trash pulls. Other than for souleater, using dark arts is very situational. I would only recommend using it on dark passenger if you are tanking a large trash pull and have blood price on cooldown, otherwise you lose mp. At your level, dark mind is the only other consideration, and that should only be when you know you will be taking a lot of magic damage.

TL;DR: Spam dark arts on souleater whenever your mp allows, and try to balance your mp so that you hover in the 25-50% range (this way you are using your mp when available, but not putting yourself in danger of dropping out of darkside).