Give the example of say an area of effect flash bang manoeuvre (which could be a grenade rather than magic) it makes sense that all enemies in the area who do not make an endurance or alertness (to close eyes) check would be temporarily blinded this would disadvantage all of those who did not resist the effects. I think each of the opponents who failed there defence should have a free tag of blinded on them which could then be tagged for a bonus or to stop them dodging fire for a turn. I know this is not the way area aspects work in the system but it seems the most real to life way of doing them.
Now I'm just reminded of flashbangs in heavy gear. That was hilarious.
GM: There are a bunch of mooks in the room, what do you do?
Player: I throw in a flashbang.
GM: How's that work?
Player: Dunno, didn't look it up, just thought it might be good to have.
GM: Alright, well it says that they have to make a very hard roll... or be completely incapacitated... They don't make the roll. What are you going to do?
Player: I'm going to go stock up on flashbangs.
But seriously I'd say if you want to apply aspects to multiple targets in combat, the best way to do it would probably be with an enchanted item or potion using the thaumaturgy model. That way it's legit and works in the proper time frame/situation.