Don't use Dark Arts on Dark Passenger on single-target.

DRK boss opener should look like that:
Dark Arts > Unmend > Plunge > HS > CnS + Blood Price > SS > Salted Earth > PS > Dark Passenger + Low Blows > Scourge

Weave defensive cooldowns between those if needed. I don't use Salted Earth before pull - since those dmg will be reduced by 20% Grit penalty, there is no need to oversweat over 7.5 sec.

One power slash is not enough to hold aggro. I am using 2-3 power slashes, then drop Grit. Once you drop Grit use Blood Weapon, then use Dark Arts + Soul Eater as often as possible.
Also, don't use Souleater without DA, until you are in Grit and need some extra shitty healing.

Once you start to loose control, wait for Blood Weapon effect to wear off and then use Grit for some additional power slashes. also, before you enter Grit, don't use Dark Arts unless buffed Dark Mind is needed. But if DADM is needed, you should be in Grit already.