Jay, if the black marks are spray paint, you need to use a solvent stripper. Let it cook 30 minutes, then agitate with a natural bristle brush on a stick, sometimes called an acid brush. Flush it all off.
Follow up by spraying a wax stripper (it'll have butoxyl in it) to help break up any residues, also to act as a grease stripper. Work that in with the acid brush, rinse, repeat until all of the shading is gone.
Then pressure wash the whole shebang and end by spraying bleach on it, to soak in and discourage mildew regrowth.
If the black is mildew, ignore the stripper part.