As I mentioned in another thread:
- Aye-Aye was turned dark by Vaatu. After being briefly turned back, he expressed no conscious awareness of his actions during that state.
- Furryfoot turned dark despite having literally no reason to. After this, he did everything that Unalaq said, even if it meant harming his good friend.
The Dark Spirits do not have free will, at least not the ones that Unalaq is shown ordering around. The Spirit that Bumi charmed just happened to be able to be reached by music, it isn't the case that it chose to go back to normal. In fact, I'm not sure if it fully reverted, as it still looked very much like a Dark Spirit.