My most successful 'black spot' treatment
When I found black-spot spreading across my bell pepper plants -- including a big black patch on the one pepper growing -- I tried every organic method I read about: spraying with baking soda, with milk, with cornmeal juice, with compost tea, dusting it with cornmeal. Milk helped a bit, cornmeal helped a bit, but the most effective method was completely heretical. I took some of the wormcastings from my worm bin, added a bit of water to make a paste, and just slathered it on the affected spots. After three such treatments, the black that covered about 1/3 of the fruit's surface is almost completely faded away.
I call this "heretical" because everything I've read about foliar use of wormcastings says to steep them in water for at least 24 hours and then just use the water.