Exactly how it works isn't very well explained. Implicitly it's channeling your emotions in tandem with aether to produce effects. The problem with that is that negative feelings tend to linger or produce stronger effects than positive emotions.

Part of the 50-60 DRK quest is getting Sidurgu to admit the reason he fights so hard for Rielle is because he loves her (in a platonic way), and that the reason he took up the sword was vengeance against the Ishgardian Temple Knights for taking away his loved ones. It's a very emotional power unlike most other magicks, which can make it very dangerous.

Now where that darkness comes from, well... Zodiark? Don't know yet.