The Cornell mixture (1 teaspoon baking soda in 1 quart water with a few drops of a light oil) was originally meant to control Powdery Mildew but a number of people reported good results controlling Black Spot also so Cornell tested that and found it did.
Also you can mix a 50/50 solution of fat free milk and water and get about the same results. Spray both every 5 days if the plants are infected or every 7 to 10 days if susceptible to prevent the problem.

The only milk solution scientifically tested was a 10% solution of whole milk in water (in both Brazil and in Australia). It was found to control powdery mildew. It was not tested against black spot.

Othr uses of milk sprays are based solely on anecdotal evidence, which is not to say that one or more of them doesn't work. They just haven't been proven by proper testing.