Perhaps a Simple DRK Solution

[Kabooa]

What leads you to believe this? Having BfB would encourage DRK to stay in Grit while MT. While in Grit+Darkside+BfB you are (+5% in +5% out damage) while in Darkside alone you are (+0% in +15% out damage). The difference is that BfB has the benefit of not costing any MP and not costing a GCD to revert back to the tanking stance. Further, you would not ever want to use BfB outside of Grit while MT as you would have +25% damage incoming. DRKs niche among the tanks is its ability to do the most damage while in its tanking stance, this change only improves upon that.

100% certain it's not additive.

With Grit on, Dark side pumps you up to 92% damage ( 80% + 15% of that 80%)
With BFB, it's +10% on top of that (101%)

With Grit on (75% damage taken) and BFB (+25% of that), you're taking 93.75% instead.

For the amount of damage you gain, the damage you take is not worth it. You can get this set up now by simply dropping Grit.

Edit: Apparently BFB got changed to 25% damage taken (or maybe it always was? Math edited!)