2nd vote for Mr. Clean Magic Eraser. It works.
Make it wet, but not dripping. Simply wipe a small part of the awning. Follow with a dry cloth. If it all does not come off, wipe again and follow with the dry cloth. Do not scrub, just wipe. Move to another small section.
The chemical in the Mr. Clean Magic Eraser will also keep mold and black from coming back for a long time. I do my awning in the Spring, usually during my first camp of the season. This way, I can use a picnic table to stand on and not have to use a ladder. (It's just the way I like to do it).
Then I'll do it again, sometime in August. But it's never really black. Mostly, it's just bugs that got rolled up in the awning.
In order to get the top of the awning, I use a long extendable painters wand and have a soft RV brush on the end. I then make the Mr. Clean Eraser really wet.... dripping wet. And lay it on top of the awning. I then take a dry towel and wrap it around the brush and simply lay the brush on the Mr. Clean Eraser.
This provides enough pressure, and does a nice job. I then simply move the brush up and down the awning. The towel drys it and also removed the grit. If there is a more stubborn spot, like tree sap on the top side, I just move the brush up and down a little longer on the same spot. It always comes off, and no black reappears.
Yes, it takes a "little" effort to do this, but the pay-off is WELL worth it. The awning stays white on the underside, except for the occasional bug poo. A simple spot wipe ever now and then will clean that off too.