When a Pokémon with Justified is hit by a Dark-type damaging move, its Attack is increased by one stage.
⚠️This Ability does not provide immunity or reduction to the damage, nor does it activate from a Dark-type move hitting its Substitute.
⚠️Dark-type multi-strike moves (such as Beat Up) will activate Justified once per strike, increasing the Attack stat that many stages.
~Bulbapedia