Black rubber trim, be it around the front or rear screen, or around the door/windows, often goes off colour/dark greyish.
 
My usual go to product would be the back-to-black type of product – you know, the black gooey paste that looks great when it goes on, but it never really dries, and comes off as soon as you wash the car.
 
I stumbled across this product from Germany:
 
GummiPfleger – it translates to “Rubber Protectant”.
 
It goes on almost clear, and dries, and doesn’t wash off on the 1st wash.
It’s not cheap, but it is good.