Really nice game. It's pretty spooky, and I love the mechanic. It was especially interesting when I noticed elements of the dark world bleeding into the normal world. That was a very intriguing moment that hooked me and I wanted to find out more.
The double jump isn't tutorialized well. Despite having read it, I had already forgotten about it at the first jump. I don't tend to remember things I read. So several times I died on the spike trap trying to get it in a single jump, which had me thinking "Oh.. This must be one of 'those' types of games." Personally I think it'd be better to introduce the double jump 1) after the player understands and has played around with the single jump, and 2) in a setup where they're not in any danger. Like with some terrain jumping.
There's a bit where the terrain drops off into a spike pit. But there's no visual difference between the hole and the terrain that's safe to walk on, other than the spikes so it's hard to see. Personally I felt a little cheated because of it, but that's a bit of personal taste.
Overall great game. I would love to see this concept expanded upon. This has inspired me to play around with masking in Godot.