Hot-take:

I'd kind of love it if Blood Weapon and a defensive or two (with alternate payoff and thereby some banking opportunities, a la TBN) were our only MP spenders and MP spent in turn generated a separate... Shadow gauge. Add some interactions between Shadow and Blood and you've got your thematic "Black Blood" burst periods. Drop Quietus; make Abyssal Drain a variable Blood spender, and add a leech (damage dealt heals you) period to Bloodspiller, causing all Blood spenders to self-heal. Have a low base/resting point for Blood to which DRK will passively generate gauge when out of combat (e.g., 30 out of 100 Blood, enough for a single undercharged Abyssal Drain).

Living Shadow is unlocked after having spent X amount of each among Shadow and Blood. Rather than displaying yet another gauge, you simply see Living Shadow's 'cooldown' spin up to show the duration it'd have. While Living Shadow is active, your damage is reduced by 20%, but your Living Shadow precisely copies your offensive actions for half effect. As such, it's still a damage bonus (.8*1.5) of 20% -- effectively more when counting MP generation. There, we'd now have the best doppelganger skill.