Only buy a Black car if you like cleaning, I have owned Black cars for a long time and they are really hard work to keep, but the satisfaction you get when they are clean is worth it.
I suggest you try one of those paint protection systems, they cut down on polishing which causes swirls and ultimatly spoils Black paint. I have Toughseal on mine and the car was Blacker after it was applied then when I picked it up from the dealer.