DRK doesn't need a second combo. Combos lock you into a fixed rotation. Additional combos are always some variant of base combo/DoT upkeep/buff upkeep/debuff upkeep, which you end up just cycling through in some fixed ratio. Does this metronome-style gameplay challenge your conscious decision-making in any way? Not after about a week.

The Blood system presently is superfluous. It would be fun as part of a system in which you trade-off MP between blood to generate more and more resources, but that wouldn't be intuitive to just pick up and play.

I really like what's being proposed for Reaper. From the sounds of it you can enter your burst window on demand, but it powers up based off of how much you charge it up first. This type of 'resource-gated' burst interests me more than our current 'time-gated' burst windows.

You could do something similar, like this:
1) Get rid of blood. Using Edge/Flood or mitigating with TBN increases your Darkside.
2) Darkside buffs your damage and haste while active by some fixed value.
3) At 50 Darkside, Bloodspiller and Quietus become unlocked as fourth combo actions, to generate even more MP and do even more damage.
4) Use Living Shadow at any point to execute some burst damage depending on how much gauge you've accumulated. Your meter resets after use.
5) You can use a finisher to terminate Living Shadow early and regain some MP and Darkside to start building again.
6) At 100 Darkside, Frey tires of this charade and shows up regardless of what you have to say on the matter.

I think that the problem at present is that every job in the game, regardless of whether it's a fixed rotation or resource-management focused, tends to play out like clockwork. I really just want to see something that just has too many slight variations to spreadsheet out and requires on your gameplay sense to succeed.