I think part of the reason is that a black surface gives the highest possible contrast ratio of light and dark areas, so, if it's not perfectly flat there will always be areas that scintillate reflected light, highlighting even the most microscopic irregularities in the the surface. Unless it's moulded in a perfectly blemish-free mould, which is flat from the beginning, you will only scratch the surface if you apply polishing techniques.
Perhaps Jeweller's Rouge might be worth trying and is not expensive. My grandad used to take the scratches out of glass and plastic with it; he did my watches a few times, which usually had plastic watchglasses, when I scratched them. I have some coming shortly and will have go scuffing a black plastic object and see what happens using it.
https://www.amazon.co.uk/gp/product/B00O3GVV2I/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1