I think this video will be useful for what I'm trying to achieve.
https://www.youtube.com/watch?v=Iwc0UZgDiGg
I'm writing this out for my own sake, as well as for anyone that might stumble upon this in the future...
I'm going to cover my room with a dark, semi-transparent rectangle. This will make everything behind it - objects, sprites, backgrounds - darker.
Then I will create a new sprite that will follow the same path as my sun/moon object. It will simply be a circular gradient with no color, just transparency. This will draw on the same surface as my dark rectangle and will therefore cut through the dark rectangle and display the normal objects, sprites, and backgrounds as they are without the tinting.
Not totally sure this will work as I picture it, but I think it's a step in the right direction. Perhaps this mixed with a gradual change in background color will result in a good look.
In the video, if you skip to the end and look at the guy's torches, this is the type of effect I am interested in.