Black cars show dirt easily, end of really..
To make it easier to clean, get it professionally detailed/paint corrected and then ceramic coated. Not cheap but dirt will wash off much more easily. Apply a secondary wax coat and then wipe the car down each day with a good detailing spray to keep the dust off. (as long as it doesn't rain)
Oh and stay away from those hand/auto washes if you want to keep a shine on it.
Black cars are hard work