If you want a black background and have onion skinning work, you need to set up backgrounds correctly. Right now, you just made a layer with a black box and said it’s a background but that’s not the right way to have a colored background.
Use this option instead:
This will turn your last layer into a proper background and you can change the color to whatever you want. Then you will be able to have onion skin work ontop of it.
Not sure if this is what you want to do but it’s an option