I recently washed my car and later waxed it with Turtle Wax for protection. Unfortunately I also waxed some of the black plastic parts like the bumper and antenna. The plastic is a little porous and the wax filled in the pores - in white of course.
I tried hard to wipe that out and also a “plastic black” spray, but to no effect.
You might try a solvent, like Opps or lighter fluid. Better test any areas to make sure it doesn’t harm the plastic. Another idea is bug and tar removing products.