I updated my post with more details. I wrote it late at night so I am sorry for the misunderstandings.
->TBN not costing MP is there to ensure you can use it on CD and make good use of its 15 sec CD. When TBN breaks, Dark Arts would buff the next Edge/Flood to grant healing equal to damage dealt. This will help in dungeon pulls and offer DRK the option to recover after a tank buster. TBN doesn't grant a free Edge/Flood, it just makes the next ones heal you as mentioned.
->Oblation doesn't have to be a part of TBN. It can be its own ability to grant 15% damage reduction. The reason why I see them being separate is so that you can use TBN and 1 stack of Oblation on yourself, the other stack of Oblation on your other tank.
->Dark Mind shouldn't be 20% specific damage reduction. Take a part of its damage reduction and put it on Oblation ( 10% -> 15 or 20% ? ). DRK has to double weave for 8 GCDs during burst, weaving defenses as well within those GCDs isn't fun, if anything you have to pop them before ( and they don't last too long ) , or you delay your damage. Instead have Dark Mind convert all shielding into healing with a slight bonus. If you need healing instead of a shield you can use TBN and heal yourself for 25% of your max HP. Shield healers can also help you. This will work very well with TBN being free, and allow DRK to deal with auto attacks too ( for which a 7s shield doesn't last long enough for usually ) Your Edge/Flood's healing can be influenced too.
->With TBN costing no MP, you can no longer do:
TBN on yourself / other tank -> BW -> Hard Slash -> Edge + Syphon Strike -> Souleater + Living Shadow. You will lose on 600 MP from BW.
Instead if your plunge grants 10 blood, you can:
Plunge -> Hard Slash -> Edge + BW -> Syphon Strike -> Souleater + Living Shadow. You won't lose on any MP and still get 50 blood.
Or any trait that allows you to get 50 blood at your 3rd GCD is fine I guess.
For the Delirium idea, I just thought that instead of spamming Bloodspiller over and over, you could have another combo that costs blood gauge per action so you use it when you are at >70 and Delirium grants the use of that combo for free.