I've found that those magic sponges are really good at getting stubborn black marks off. We had some particularly bad ones round the flap to the battery box on our Pathfinder and I tried alll sorts of chemicals to get them off but the only thing that shifted them was the magic sponge. No effort at all.
I clean boats for a living - first of all you need to get the loose crud off with a wax shampoo. The best thing to use rather than chemicals (which could destroy the paint work) is polish, any car polish will do. Polish & buff up the body & not only will it look great but it will add protection against the elements.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.