The Nightsister NPC from CotR has an ability called "Mother's Blessing" that allows them to force opponents to reroll blank Setback:

Quote

Mother's Blessing: A character who targets a Nightsister with a combat check must reroll all [Setback] showing a blank face.

My question is, how many times do you reroll them? Say you roll two Setback, and one shows a blank face. You reroll it, and it again shows a blank face. Do you reroll it again, or is that it?

FFG games are pretty consistent about re-rolls, in that they only apply once. I can't find a reference in these rules though. As a GM I'd only require one re-roll. Besides, if the intent was to guarantee a non-blank result, it would have been simpler to just let the targeted character remove the setback and apply either failure or threat per setback removed.

Re-roll once is probably the fairest conclusion. However, I would run it the other way. Re-rolling blanks until a face comes up is far more interesting.