I honestly dont know what you'd do to Oblation to make it feel good to use, I don't even really think DRK needed another mitigation only move at all really

Maybe have it also give a Dark Arts like TBN except without a condition and move Abyssal Drain over to being a no CD move that can be cast with 3000 MP or a Dark Arts? Thats the most I've been able to come up with DRKs current design.
You'd probably have to adjust the heal potency on AD to make that even worth it, maybe they could do that thing they've started doing where it has a higher potency on the first target and a lesser potency on every other target.