Why did black spots appear all over my roses after spraying with Cornell Univ.'s baking soda recipe?

I sprayed all my roses with Cornell 's recipe baking soda, horticultural oil, and water in the very late afternoon. I made sure they were sprayed very well and tried to get the backside of the leaves. When they are dripping, I know they got a good coating. The very next day, all (and I mean all) the leaves were covered with small black spots , shaped like circles. Where did they come from?