If TBN's precarious balancing will make it a problem with competing with DA, then they will probably need to do one of three things:

1) TBN instead costs Blackblood to use and doesn't grant anything else. It's just an on-demand cooldown like Inner Beast and Shelltron. I would say the best solution, but it becomes a very underwhelming capstone ability, and doesn't solve the problem with defensives before 70.

2) TBN is just a straight cooldown. The simplest solution, and it helps with a cooldown gap. It's just a very boring one.

3) TBN is only available when in Grit. Kind of like how they split Blood Price and Blood Weapon to depend on whether Grit is active or not, they could make TBN Grit only and DA non-Grit only. TBN's MP cost would come down to match DA's price, and MP gain can be put on Bloodspiller... maybe. I think this could be the most elegant solution but also the most controversial.