Hi, Well I've never heard of the cornmeal method, but doubt very much it would do anything. The other method might work, but I don't recommend mixing your own solutions because if you get the ration incorrect you can do more damage than good. Also, I don't know if it would work. I recommend two things for Black Spot. 1. Don't grow rose varieties that get the disease. There are many new varieties that have natural resistance. 2. Use a commercial rose fungicide. Black Spot is a fungicide so use a product that's designed professionally to combat the disease and follow label and application rates on the label.