I have had extremely good results from removing plastic parts (where possible) from the car and then spraying them with satin black paint.
The finish is pretty much perfect and on my Defender the front light surrounds are still looking like new three years/36,000 miles later.
I also did the wing mirror casings (on the Land Rover) as they had faded to grey after thirteen years, and now they look new.
I have found that the various polishes and black dye products are only a temporary fix.
I have also used this method on parts like the scuttle trim and underbonnet fuse box cover on my Clio 182, again with excellent results.
I generally spray the parts in the garden on a sheet of cardboard to avoid fumes from the paint.
Just be sure to clean/degrease the parts first.