Black is beautiful, no doubt, but almost impossible to keep clean. I own a black Trans Am and she is the best thing to look at clean but keeping it that way is almost impossible.
Things to consider when purchasing a black car.
1) Never...NEVER allow anyone to detail your car, even the dealership. Black is NOT forgiving, and if someone else cleans your car with dirty rags or even cleans with dirty towels expect swirls. The only way to get rid of swirls is with a filling type of wax which hides them or to use an abrasive polish such as 3M Swirl Mark remover to remove. I always clean my car and use 100% American made white cotton towels to wash and dry and still to this day its almost impossible to avoid getting some micro scratches/swirls. This is a fact of life and black cars accentuate this problem.
2) If the car is to be a daily driver, rock chips are also a fact of life. This is inevitable unless the car is going to be a "garage queen" and not see the road that much. Rock chips are frustrating as hell and there is nothing more frustrating after cleaning your car then seeing little white specs in the front where debris has hit the car and chipped away the paint. This again is a fact of life and unavoidable unless you plan to put a bra on the front end, which isn't always the best idea.
3) WATER SPOTS...be warned...they are also a killer on black cars. I've always been carefull of these on mine and know not to wash the car under direct sunlight and try to wash the car after a rainy spell as there are mineral deposits in water and when it drys on your car you get water spots or sometimes called "water etching". Somtimes a clay bar can remove these, sometimes not and at that point you'll have to polish them out. I've had to polish a couple areas on my car before as a trip to Tulsa one weekend my brothers sprinkler system got my car good, something about that Tulsa water.
Other than Water Spots, rock chips, and proper cleaning techniques the only other thing I can think of right now to watch out for is bird poop...you let that sit on your paint and your asking for trouble. Very acidic and can cause discoloration in your paint.
If I were to buy again, which hopefully i'll be getting into an 05 Stang soon, I won't be getting black but probably the grey as lighter colors hide these daily driving imperfections better. If you are to go with a black color here is what I would reccommend.
1) Always detail your car yourself.
2) Never go through a car wash
3) Detail prep, most important part. (dish soap to strip off all wax...dry, clay to remove all oxidation and rail dust...then dish soap wash again...dry)
4) Wash with 100% american made cotton towels (remove the outer edges of the towels)
5) Always separate your washing towels, drying towels, and buffing towels and always use 100% american made cotton. Never mix em up. Never dry with dryer sheets.
6) Never wash in the sun, use a shady area.
7) Personally don't like chamois, sometimes they can retain dirt and when drying can scratch your car.....and if she's black you'll really be upset.
8) When washing try not to go in swirling motions (good idea for black cars).
9) use a sponge for tires/ tire wells...never using your towels there.
I'm sure there is more i'm missing but those are some of the big points. I can tell you that for me the biggest difference was prep...doing the clay bar to remove all oxidation and rail dust to get a squeaky clean surface. I personally use Zaino and believe it to be the best product out there.
Just remember, with black there is a LOT of work to do to keep the paint tip top if your into that. Hope the info helps.