The ally combat effect neutralization of Impenetrable Dark only activates during combat involving the unit. It has no effect if the unit is neither the attacker nor the defender of the combat.
The ally combat effect neutralization of Impenetrable Dark does neutralize after combat effects such as the HP recovery prevention effect Noble Bow and Blood & Thunder.
The ally combat effect neutralization of Impenetrable Dark does not prevent 【Savior】 from being triggered, because 【Savior】 is not a during combat effect.