When Chatot and others make mention of a heart (soul) being touched by darkness, it isn't just an expression. It's an actual thing that happens. (In CtS)
A heart being touched/enveloped in darkness occurs when a person is feeling an emotion so intensely, that their mind and body cannot handle it. They are placed in a state of mostly instinct, and that emotion. With a complete singular focus on maintaining the state and accomplishing their goals. Similar to Primal Dialga. Just still being partially able to (though unlikely to be willing to) be reasoned with, as well as Primal Dialga being a special case.
It can happen with any emotion, though its easier with some emotions than with others, and mostly depends on the personality and morals of the person in question. Like with Happiness being twisted and corrupting the individual through greed. A survivalist being corrupted with a fear of death, or disregard of their own life.
To someone with a lack of morals, it really comes across as a power boost. And it somewhat is to an extent. The singular drive often results in someone being able to fight for far longer than they would be able to normally. Muscling through things that would knock them out from shock or pain.
It's also possible for someone to 'lean-into' the darkness for even more strength. Either by embracing the lack of inhibitions, or just not resisting. Such as with Dusknoir. His belief that he had killed his daughter sent him spiraling into such deep grief and self-loathing that he felt no reason to even question Dialga's orders. Much less to try and get attached to anyone.
It is possible for someone's heart to be purified. This happens to Grovyle when he pieces together that Sunny is that human that he was practically an older brother to. He still has his overall goal of ensuring the future's existence, but he now has much more personal reasons to get involved as apposed to altruistic reasons. As well as helping him form his outlook on life that eventually convinces Dusknoir to fight for the same future that Sunny would want.
There are others too. Drowzee got corrupted by greed, and was early on in it enough to later be relied on for help. Darkrai got corrupted by overwhelming loneliness turned hatred.
Sunny, having been Darkrai, is able to almost 'sense' when someone's heart is corrupt. Feeling like a sense of dread or a pit in her stomach the closer she is to someone corrupt. As well as the strength of the darkness being a factor.
Most anyone who's had their heart corrupted is able to have a feeling about it. But Sunny is extra sensitive. Due to something happening while she was Darkrai that was so traumatic, it permanently scarred her subconscious.
