My suggestion:
Let Darkest Night be Dark Arted
Make it restores mana when popped.
If used in single form let it restore 3000 a little more than it cost. This makes it a small DPS gain.
If used under DA let it restore 650ish mana per bubble pop This means if all are popped It is a gain of mana and meter but if careless and not all pop it will be a loss. This adds depth dps and utility
You could add a 15 second CD to make sure it isn't spammed
These numbers would need careful thought but it took me 2 minutes to think of this and it would take DRK from being garbage to being perfectly fine with slightly increased DPS, mitigation and raid utility.
I also feel they should give back slash buff to DRK as right now PLD + DRK means both do 10% less dps than if a warrior were present and since PLD is considered a must DRK is not wanted or needed.
Please do SOMETHING SE. Don't leave DRK useless and broken for another patch.