On my Mazda and probably the same for Rodder, there is a painted area around the rear windows and the upper door that is flat black.
There's no plastic involved, it's sort of a two tone paint scheme done by Mazda.
It's separated from the rest of the body by an aluminum strip backed with a piece of plastic.
Mine is kind of tired, the black is oxidized a bit and doesn't look black anymore.
I tried to cut mine, but it didn't help much, it looked OK at first, but after a few days it looked just the same as it did.
What I would do is go to your auto parts store where they can mix up some flat paint for you that isn't a primer.
My auto store (not a chain) can make spray bombs of any colour I like, including flat black _ a couple of cans should do it.
Some 400 grit paper and some carful masking should do the trick.
If you have the aluminum finishing strips and the plastic backing, the plastic doesn't like being disturbed, it shrinks as soon as it's disturbed, at least that's what happened to mine.
I'm going take a heat gun and gently soften it so it will form around the aluminum and be properly mounted once again.