Just plain micronized sulpher in solution is fungicidal. No need for soap. The combination in summer heat might burn as each ingredient by itself has a good chance of that.

Good control of Black Spot on roses can be accomplished with either a 50/50 mix of fat free milk and water or 1 teaspoon of baking soda, and a couple of drops of a light oil, in 1 quart of water, applied every 5 days until the BS is gone or as a profilactic spray every 7 to 10 days before the plants get it.