I'm not an expert here as I only detailed and restored cars for a decade, and that devolved into boutique detailing for select clients long ago (customers are the worst!)keeping black plastic looking black is a nightmare. I feel like I go through more of that meguars ultimate black more than any other cleaner
But your problem is that you're using a garbage product and wondering why it doesn't work. At least you didn't say "Back to black", which is possibly the only shi%%&^# product you could possibly be using to restore or protect black plastics.
If you want to actually re-finish black plastics, may I suggest Solution Finish. This stuff actually penetrates the plastic and will restore it's color. I've even used this without any surface prep work on a neighbor's stupid JEEP with fender plastics that had turned to a white powder appearance. Penetrated perfectly on the first pass and they still look good 5 years later.
Solution Finish: Black plastic and vinyl restorer
I have been quite upset about the only hard top on the 2-doors being grey. So if I have to get stuck with that then i'll be trying the new Solution Finish for grey plastics.
Solution Finish: Grey plastic restorer
Also, using a UV protectant such as 303 aerospace protectant will go a long way in preventing the fading from starting.
FYI: The small, $15 bottle of Solution Finish Black should be enough to do about 20 Jeeps, restoring every single piece of faded black plastic and fender on every single one of them. That stuff just keeps spreading like a champ!