Well, there's a couple ideas I can see on BLM's end that could be shifted to RDM as part of their hybrid nature.

Verum Oratio (Enochian): Grants the RDM a SpS buff that stacks up to 5 times as they cycle through Verfinishers, probably capping out at 10% or so. (Gain 1 stack each time you use a Verfinisher, 45s duration)

Aetheric Reversal (Transpose): Swaps your White and Black mana totals, on something like a 60s cooldown.

Either or both of these might be terrible ideas, but they're the best I could come up with for how to incorporate more Black Magicks into RDM. Thoughts?