I love my black sapphire e90. I try to wash it ever weekend but I'll wash it at least every other weekend. I used to have a white car but its just no fun. You wash it and it just doesn't shine or look as wet after waxing or anything like a black car. I've even gone 4-5 weeks without washing my car once in winter and it didn't look that bad other than the brake dust build up.
Just buy a buffer and polish the car yourself twice a year. Once before summer and once before winter and you'll be fine.
One bad thing about black cars is that the dealer really knows how to mess up the paint when you take the car in for service and they give you courtesy wash.
I have to tell them not to wash it now.