First of all, delete Oblation.
Once you activate TBN, it will proc Dark Arts right out of the gate. Once the shield breaks, you will then get 10% damage mitigation for the next 10 seconds. It still won't be as powerful defensively compared to the other tanks, but the damage will make up for it every time. There will never be a reason to not use TBN with this change.
You could make it a level 82 trait to upgrade it, or you could change it entirely. Also I'm not sure if this buff would constitute a longer cooldown or not. Thoughts would be appreciated.