Given that mirage is supposed to be in CONTROL of light and darkness with her other abilities, get rid of the response to light ability 3 and make it like this maybe.
3: press: Create a "dark light" which darkens the area , all friendly targets in small area of effect get X damage reduction
(maybe also makes enemies partially blind as if you were stealthed so they swing /shoot at nothing in the area.
3: Hold : Create a light column: All friendly targets within SMALL area of effect get Y damage increase.
Maybe also on cast blinds close by enemies like gara's passive blind or excaliburs flash of light But it's a very small area of effect.
X and Y will need to be tuned to be reasonable.