I think that it is okay, but I don’t know if you had something special in mind that you yourself currently feel being off, because we can’t know what you think. But as I said, in my eyes it is okay right now!
My only issue is that I find the white borders annoying, especially since I want the atmosphere to be mysterious and spooky like the background, but the white corners ruin the picture. From my perspective (as someone who has read many manga stories), I feel there’s something wrong with the page design .
If you have fun with something playful, doodle a little story of whatever around the border, stick figures would be sufficient - or either make it black, respectively cut it off.
I am planning to enlarge the background in order to get rid of the white border, but currently I am drawing the next page. After I finish, I will make adjustments (luckily, it won’t take long, I just need to draw a mouth and eyes, focusing on shading ).
Normally you would not draw directly up to the edge of the paper/canvas for manga because it can be problematic with bleed and crop (in print). However there is no rule you can’t do this, just make sure nothing important is at the edges (and when it is not to be supposed to be printed ever, it doesn’t matter at all). And there is also no rule that says you can’t have the borders black as if you would “print on black paper”. I read plenty of mangas and comics that did this sometimes either for dramatic effect or if you simply need all the space you can get for a scene.
The image I shared is supposed to show “how it will look after cutting the edges of the print.” The original shape has larger edges, especially at the top, while the bottom has a more extended design. I cut it to see how it will look later.
Well my point still stands. There is no reason why you cant have black borders or draw up to the edges. The reason you rarely see print do black borders all the time, is the added costs for printing (more ink) and also harder readability of the panels some times but I own at least one that actually does this through an entire book, black borders all the way. When it’s just a few pages and it adds to the dark mood then I can’t think of a reason why you wouldn’t get away with it.
However, I plan to extend the border instead of just using a black background. Also, when I take a quick look at the page layout, especially the next page, I think it would be better to extend the background to match the entire page, because the next page will be entirely gray, and a white border would be visually jarring.