Another change I think would be helpful for Dark Knight's sustain could be this:

The Blackest Night shield when fades (either broken or expired) grants a regen (whatever potency seems balanced) for 10-12s.

So no matter what, you aren't punished nearly as hard if shield doesn't break, and it helps the DRK's sustain regardless. That's if they want to just throw in a simple and quick band-aid regen for the job without adding new abilities/completely changing how the job functions. Is it super inventive? No, but imo it would be better than the literal nothing DRK's have for healing.