Well, it manifested long before she became a dark knight. After fleeing to ishgard she trained as a machinist, no longer wanting to trust in magic or higher powers she wanted to deliver some cold hard justice. It was pretty nasty and manifested as something volatile and negative, especially when around those she really hated (Lolorito, Thordan, Lahabrea etc). She had no training but thanks to her friends she never really lashed out. But her and Esteem are one and the same, Esteem reacts to how she feels and enhances it, so it's not really changing her or tempting her. It can manifest in times of internal turmoil to guide her through it or in combat as a shadowy doppelganger of Sophie. She can tap into other people's darkness and in return let them experience a piece of hers. But she rarely if ever does it, she prefers using her words.