The dark black powdery stuff is sooty mold. It grows in the sticky substance given off by aphids, scale and whiteflies. I would swab the siding down with a product call Krud Kutter to help soften the stuff and then power wash it. If that does not work, swab it with mouth wash and scrub with one of those Mr. Clean eraser sponges. Best And Happy Yardening, Nancy.