I think reinforcing that identity is likely the most logical course of action now.
I was thinking of adding more charges of Dark Art too (maybe being able to hold up to 2 as well) and change Shadowbringer to be a Flood of Shadow+ when you have dark arts.
Though I considered fusing Oblation and TBN to an upgrade at 82, making TBN's shield reapply itself after being broken, with a -5%, up to 4 or 5 times. Each time giving a charge of dark art.
Maybe when the duration expires, you'd get MP back proportional to the amount of charges left too ? I feel like that'd make it pretty strong and in line with the other CDs. (Plus, replay the Oblation visual effect each time the shield reapplies, 'cause it's a very good looking effect)

For the sustain stuff though, not quite sure. Dark Missionary should increase healing received like RDM's Magick Barrier (after all a missionary is often a healer), and maybe do something with dark mind. Was thinking of making it a MP regen button, but maybe there is something else to do with it.

Otherwise, blood weapon HP drain. But since you're supposed to use it whenever it's back from CD, it may not make it a very flexible healing tool.