I have used "Black Again." It works, but it is only temporary. You have to reapply ever few months. There is another product called "Forever Black." It is supposed to be permanent, but I have never used it. Both are available from properautocare.com. Good luck.
Forever Black is more like a dye while the other are more like durable protectants. FB is the way to go if you want a cure that's a bit more permanent.
A friend of mine says if you put a hair dryer to the grey area, it will return to black. It supposedly pulls the oils from deeper in the plastic. I don't know if that's true, but he showed me his Wrangler's fenders, and they look like new.
A few weeks ago I cured some 8 year old black plastic door handles of their greyness by cleaning them with goof-off to remove the wax that was on them. Any solvent or detergent that removes wax should work.
Then I treated them with 303 Aerospace Protectant a couple of times and they look like new.