During boss stagger, a boss is considered under the effects of all CC including stunned, frozen, immobilize.
Devouring blaze’s second part “If they are also Immobilized this bonus is increased” does not work. The multiplier remains at the base.
Similarly the Control glyph 1.2x against Stunned or Frozen is not applied during a boss stagger.
Control Aspect works during the boss stagger phase so I would assume Control glyph and Devouring Blaze should work too
Here’s another user reporting the same thing: Sorcerer Devouring Blaze Passive not Applying to Staggered Bosses