Chances are they have more of a mix of tp and mp skills, with a tp combo giving some mp back just like riot blade. Maybe also a CD to give you a more substancial mp recovery for panic situations.
MP skills will be you big threat/damage skills, and your TP your basic and MP recovery skills. That actually makes me wonder if it won't work a bit more akin to Aetherflow stacks (except, perhaps, costing MP instead of restoring it?
These are my thoughts.The Dark Knight will be using his MP to draw the power of darkness. But it’s not just a simple using of your MP to execute your actions but you draw from that power of darkness and you can either use it for an attack or use it to boost your defense and you’ll be strategizing on maintaining that power of darkness and managing your MP so it’s a very unique style of play in the tank role.