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!)
