What makes TBN special?
1. It's strength. 25% shield on 15s cooldown is amazing and feels good to use.
2. The proc. Seeing the shield break and getting rewarded for it feels good.

What makes TBN bad?
1. Having to risk 500 potency (!) of damage every time you use your tank cooldown. No other tank has to deal with anything like this.

How can we fix what's bad about TBN without changing what's good?
1. Refund the mana cost automatically when the shield times out or breaks (or, remove the cost entirely).
2. Change the proc on shield break to a Heal over Time.

These two simple changes will bring TBN up to par with the newly upgraded cooldowns that the other tanks get, while keeping the ability's distinct feel and identity. Yes, this is a straight buff to DRK. No, it doesn't make DRK overpowered. The cooldown does not need to be increased to 25s to compensate - the power of TBN is still nowhere close to Bloodwhetting.

For bonus points, you can also do something to Oblation. Have it synergize with TBN (such as making the proc go off even if TBN's shield doesn't break). Or perhaps even make it a straight upgrade to TBN (with increased proc HoT, shield duration, and/or damage mitigation). The animation certainly looks cooler...