Wrinklyninja ; How does this play with Cava Obscura? Are they trying to do the same thing?
No. They might an overlapping effect, but they're trying to do different things:
Darker dungeons mods reduce the amount of light available to see by in dungeons, affecting the visuals and the ability to see stuff, including other NPCs/creatures lurking in the dark. This doesn't just affect the player: since it affects the amount of light
in the game, NPCs/creatures will also find it harder to see you.
Ambient Dungeons changes the ambience of scenes, in dungeons and elsewhere to affect the 'mood' or 'atmosphere' of the area as it were. It is a shader effect, so it changes what you see
on your screen, by modifying the picture that gets sent to your screen, changing the saturation, contrast and brightness. Since it's not doing anything to the game, it obviously won't affect anything apart from what you see the game like.
Clearly, Ambient Dungeons can darken the dungeons and so it can overlap with darker dungeons mods. The mod's default settings are for vanilla though, so if you've already got pretty dark dungeons, you should change the settings, particularly setting brightness to not be altered, so that the darkness isn't further affected.
I've seen people pretty much flame this mod because it's 'a fake' and so on, screwing up the dungeons and making it unfair and broken by letting NPCs see you but you not see them: such people are completely missing the point. It's not a darker dungeons mod, it's a scene ambience mod, and you will probably need to tailor the settings a little for what your setup is like.