The wife has an almost new 2018 Mazda 3 Grand Touring hatchback. It is black metallic. It has been incredibly hard to keep looking clean. The first wax product I tried was horrible and I moved up to my standby from Meguires that I use on everything else with great results. It looked great for a month or so but when I washed the car this morning it felt like it had no wax left on it (it has only been about 3 months). Even fresh and clean, it always looks a little less than clean. I decided to try some polish specially made for black paint, Turtle Wax Color Magic (see the link above). It was even on sale. Given the price I didn't expect a whole lot but it needed SOMETHING and at least this stuff wouldn't leave white residue at the edges. It went on easy and the original buff off wasn't too much work but left it looking streaky and slightly greasy. It was hot and humid out which always causes problems with the buffing as the wax or polish never seems to really dry fully. Anyway, I pulled out a brand new microfiber polishing cloth and gave it another buffing and it was like polishing a diamond. The black got deeper and rich looking and the shine was nothing short of amazing. I really am impressed with this stuff. Hopefully their Color Magic black detailing spray will keep it looking great in between polishings.
The wife even said the car was faster now since it passes through the air so much easier!