There is a brilliantly simple solution to this.
Use Autoglym Instant Tyre Dressing. Wash car (and you dont have to wash the honeycomb parts beyond running a sponge / mitt over the front)... rinse off... whilst it's still wet simply spray this on and leave it to dry. Perfect black finish every time and no effort at all!
If you haven't tried it you won't believe me - trust me it really is this simple and the result is perfect every time.
Go on.......
The AG Tyre Dressing works great I prefer using AG Vinyl & Rubber Care - seems to stay a little longer !
Thats what i currently use, OR you could try Silicone spray from halfrauds its in the motorbike section used to be in a yellow tin with grey top, good for all black trim, spray on the cloth wipe over and lasts a fair bit to.