I suggest changing Greater Dark Blast, it makes Utterdark Blast, something the Necrolock should be eagerly awaiting, a lot less special. Maybe let it apply one negative level as a minor version of Utterdark. The difference would be that this one has no save to prevent it. So the Necrolock has two options: blast for half negative energy+one guaranteed negative level, or get Utterdark Blast and blast for full negative energy and chance of two negative levels.