I have an H3 with lots of hard black plastic that's gone grey. I've tried several basic trim restorers (Maguires, 3M, etc.) they work but then fade out in a few months. The best results I've had is with a ceramic based trim restorer, like Cerakote or Lithium trim cubes, these kept it black for 2 years making it worth the higher price.
I would not try the torch method noted by fordor, its a short term fix and destructive in the long term.