Better late than never, here is a more detailed (and totally subjective) breakdown of the strengths and weakness of your game.
**My platforming background**
I am a novice adept of die-and-retry platformers ; barely good enough to finish all the normal levels (B-Sides / Dark workd excluded) in Celeste and Super Meat Boy.
**Controls**
They are much tighter than when I first tried the game. To me, the controls scheme was intuitive enough (especially dash on Left Shift). My main concern is that both the wall jump and the dash should be taught at a different time, inside the game itself, and in parts of the game specifically designed to master them. Trying to master them both in the first minutes of your merciless levels is too discouraging.
**Level Design**
You are good at making the player suffer, and that's not a bad thing. But the difficulty curve needs to be a little less steep, at least in the beginning. Having the player dodge obstacles in free fall then dash is nice and a lot of fun to try again and again, but not as a first difficulty spike.
The checkpoints were adequately placed. If I am right, they also reset the "black curtain", which then starts from that defined point when you respawn. That's a nice way to avoid several issues, such as a player getting too much of a lead on the curtain (it's not gonna happen), or a player getting the checkpoint when almost crushed by the curtain (more likely) and unable to continue because the curtain respawns at the same time as him.
Another hard difficulty spike was the part in the beginning where you have to go up and climb several platforms while the curtain goes down. That's a little disheatening, since one mistake ruins your chances of dodging the curtain once above, even if the view is cleared when you make the mistake. Late punishment tends to hit harder. On the other hand, and as I previously said, you master the big design rules of a great die-and-retry, so keep it up !
**Visuals**
The first thing I would do is find a way to dissociate the black/grey particles that kill you and those that do not. Having a particle-based visual design is great, but it should not alter the readability of your game. Aside from that, I would keep the black and white visual design, while colouring important elements to make them strike the eye more (the player/a boss/a switch/a goal/???), but that's what you already do. In short, keep the dark aesthetic, but throw in small but memorable touches of color (one non-dark color by area ?).
I'm finishing this feedback in a hurry, so I might be forgetting things. Do not hesistate to contact me if you have more questions !