I suppose the real test would is does it stay black for a few weeks or longer term??

Do you have issues with dogs licking your car!!?? 
 
I have previously used "so easy" tyre dressing on black plastic successfully, applied to a rag then to the plastic.  I have found with plastics that are quite dry/faded, you need to keep "feeding" them the product of choice, ie they fade again relatively quickly after first application but you just keep reapplying.  Then after a while the shine remains for a decent amount of time and a quick wipe brings it back great.
 
In terms of polishing plastic lights et c, you can use car polish or I got this stuff:
 
Meguiar's PlastX

 
Can work with just a rag but for major fade a power buff works quickly.  Brings up plastics like new, eg faded tail lights and indicators on the SW20.  I will post some pics in my showroom thread.
 
Cheers
 
 

I'd say that would be a temporary fix, would be the same as wiping a bit of tyre shine on the rubbers.

My dog likes peanut butter too much to try this!  Don't want him to start chasing cars!

I was going to order some Black Wow and thought I'd try this trick on my 90 centre panel (honeycomb type) first.
 
It worked great! I should've taken a close up beforehand but here's the result,
 

 
This is a 24 year old piece of plastic and was looking very grey and faded, especially around the MR2 in the middle. Pretty happy with the result, lets see how long it lasts!