If SE wanted TBN to be designed to be a break even then the simplest solution would be to have the shield break and return the 2400 mana rather than returning 50 blood, then the skill would just make sense to use in all cases.
DRK seems great for prog because in grit you have this wonderful mitigation that increases your dps while helping you be a turtle. Then when trying to maximize our dps we are in a situation where it is questionable for us to use mitigation, adding additional strain to healers who are also trying to push their dps and optimize. Conceptually this is just weird, it is a dps increase when you want to play defensively, and a break even when you want to play offensively. I play drk to play an active tank, what I dislike about dark is that to maximize the class you need to give up so much to break even with the other tanks.
I drop grit: I lose 20% mitigation, 1200 mana restore, a self-sustain in the form of soul eater, and one of my core mitigations becomes questionable/needs babysiting. We gain access to one dps cooldown in the form of blood weapon. Now maybe we can make the arguement that we lose the bonus 1200 mana restore because blood weapon is a large resource generation, but I feel we might go cross eyed there (I lose resource regeneration because another button does that now every 40 seconds... ok), and nothing replaces the self-sustain or the drop in TBN mitigation. I am also aware that pld and war also lose things by going dps stance, but I think the gains far outweigh the loses in these cases.
IMO (not worth much I've tried selling it but I'm going to say it anyway) a main tank should not have to skimp on mitigation to such a degree to maximize dps. I play this class because its more fun than the other two choices because of its active style and at the end of the day thats what it is about, but similar to the fending/slaying argument I don't want to have to choose between mitigating damage and squeezing in an extra siphon strike or soul eater, and to my knowledge no other tank needs to make a similar choice.
Other things:
The cooldown time on Shadow Wall does not make sense, an additional effect, longer duration, or be on a shorter cooldown are all viable things.
Sole survivor needs a single target effect that doesn't depend on the target dying, especially when there are so few adds in the raid tier, v3s being the exception. Personally I would like to see this convert damage to HP/mana/blood.
Dark passenger needs an additional effect to justify the large mana cost. Perhaps it could act as a personal crit buff, act like a spear (the cooldown reduction version) for dark knight skills, or allow for blood gain from other GCD actions.