I absolutely LOVE the look of a black vehicle... until I get close to one and am reminded of why I plan to never own one!!!
That being said - keep in mind - 100% of all colors do the same - the difference is - the darker the color, the more those swirls and scratches show. I have owned a dark blue car - and it was nearly impossible to keep looking good - which was the final nail in the coffin for me and dark vehicles.
BUT - as someone else posted - the best action is going to be putting on coats of the best sealer/wax you can. Some are actually pretty good at protecting that paint/clear coat, and when they have enough coats, do a reasonable job at keeping the swirls and scratches to a minimum. I would start with a full correction, de-oil the paint surface (most polishes contain some oils that can hinder a sealer or wax from getting a good bond to the paint). Then seal with something like FK 1000P (
Finish Kare 1000P Hi-Temp Paste Wax, high melt polymer car wax, Finish care auto wax). FK is awesome, though not the most gloss for dark colors. It's protection is absolutely awesome. A preliminary coat, followed by at least one follow-up lasts for 6 months - 1 year in my experience.
In-between, when you wash - keep on with the two-bucket method, but check your wash mit for debris. It takes surprisingly little to swirl up dark paint. Of course, pollen is the absolute worst offender.
I like to use Optimum No-Rinse. It doesn't suds up to speak of, but cleans very well - and it actually helps to prevent scratches. I have used it on dark finishes with GREAT success.
No Rinse Wash and Shine
I use it for a more "standard" wash - in one bucket with water, I use it as labeled for a "no-rinse" wash, and also mix (according to label directions) to use as a detailing spray/clay bar lube. Stuff is awesome!