I've got a black truck and get what seems to be exactly what you describe. Those pesky white marks that don't go into the paint.
I've used a pre wax cleaner that was varsol based and alot of elbow grease to get rid of them. I'm not sure what there from but I belive it is car doors. I was having a smoke one day by my car and some chick hit the truck quite lightly, I didn't even know, and she came over to tell me about it. Small white mark, her car was blue, go figure. A good coat of wax should fill in any small stuff as well. I'm no detailer but I've owned 4 black vehicles and with a little work you can keep them looking as good as you know they can.
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