Try Bonide Infuse spray. Walter Reeves recommended it on his radio program. I've been using it alternated with Bayer's Disease Control for Roses with very good results. It's a constant battle though. You can never win. Good luck!

I use the Bayer advanced disease control fungicide only! spray. You can get it in Lowes. It will not only prevent BS, it will stop active breakouts. Spray it once a week for 3 weeks, then you can go for every 2-3 weeks. Don't stop until we get a hard frost, otherwise the roses will carry the bs over into next year. Unfortunately in our area, most roses will blackspot and defoliate completely.

I'm a true believer in Bayer All in One.
$16. at Home Depot.

Great! Well at least I know where to start. Also, I thinned out the rose that is the most effected. I am hoping that will allow more sun through to keep the leaves dry.