I just tried DRK and... I closely paid attention to my MP management.
And I realized TBN is much more worse than what I was thinking.

It's on a 15s technically, but a 60s cooldown in reality. Since you cannot store more than 1 Dark arts and are supposed to keep your MP until next 60s, you are not encouraged to use multiple TBN.
Am I right? At this point, TBN should straight up be a simple shield on 15s CD.

If SQEX absolutely wants a rewards if you break it, why not make Dark Arts impact defensive or 60s cooldowns?
Between every minute, you need to break 1 TBN (available every 15 seconds, therefore around 3 occasions) to get Dark Art and empower your 60s phase.

Oh and buff Oblation. 10% is weak, 10s is short for 10%, 60 seconds cooldown is ridiculously high, I have not found uses for the second charge.
Hopefully BW on stacks like Delirium is coming to 6.1.