Hi, first I apologize as I can not post pictures for some reason to prove this but it is absolutely true. The Devouring Blaze Passive is not getting applied to Staggered Bosses. I know this for certain because I have compared non-critical hits on staggered burning bosses with critical hits and a +6 devouring blaze passive. +6 devouring blaze gives x150% increased damage so even with damage variance all that is needed is a comparison of one hit with one crit, the innate damage variance in the game is not enough to skew the results considering how large a x150% multiplier is.
Again I cannot post the pictures but this is easily replicated by anyone with +6 devouring blaze. On multiple bosses (yes they are burning, I use meteor) I will usually get white hits of around 200k and yellow hits (criticals) of around 500k. My critical strike damage is 150%. If devouring blaze was getting applied I should expect critical hits of around 1.25m.
I have observed this over and over after hours of play in different instances and vs different bosses. I have even tried to use different sources of burning in case there was some sort of issue with meteor specifically. Same result every time.