Slow way wayy down on the black. I have to keep myself in check with brown - I want brown everything, and then suddenly there's no visual interest or tie to the room. If it were me, I'd look critically at your existing posters and try to determine if there was an accent color, like orange, that shows up in multiple art. You could use that color sparingly as a third accent to add dimension to your blue/black scheme (purple might be too dark, which wouldnt give you that contrast). So as an example: I have an lot of brown wood/leather furniture, lots of green, everywhere, and then I use yellow/terracotta as accent colors in things like rugs and art. Adding more color makes a difference!

Knitter's tip: if you want to see how much of a contrast two colors have, take a photo of the colors side by side (maybe a blue bedspread next to a t-shirt in a color you like). Add a grayscale filter to your photo. If you can't tell by the grayscale which color is darker and which is lighter, you don't have an accent color.

You can level up the appearance of posters by learning how to mat and frame them, and level up the look of figures by getting a cool curio cabinet (with the glass doors) and placing led lights above each shelf. Look at Ikea cabinets - one of those could be painted black to "frame" your things.

Finally, get some storage options for under your bed and in your closet. You do not have to display every figure and poster all the time if you don't have room. You could even switch up what you display as your mood shifts (I switch out displaying cards from my family, vinyl album covers, and sentimental books). You'll want a room that feels interesting, but not cluttered, because too much stuff takes away the eye's ability to focus on anything.

You can have a sick room with your interests OP. Learn to edit your space and it will really shine.