The RBF does reduce Backlash damage from Decay. With a little more time, it would probably be easy enough to work out how much AMF Backlash Reduction the RoBF provides.
With 100,000 HP, and Decay facing a 1.00 AMF I took;
Without RBF: 20,002 damage
With lvl 113,430 RBF: 17,441 (a 13% reduction)