There is some research that indicates the spores that cause Black Spot do not live in the soil very long, about a month, but your best defense is to provide a soil well endowed with organic matter and then mulched well so any water hitting the soil could not possibly splash any spores there may be onto the plant.
I have also used a 50/50 fat free milk and water spray to eliminate black spot on a new rose from a nursery, did not see the sighns when I bought it.

In my garden the best organic treatment of the soil against blackspot have been seaweed meal / liquid seaweed, cowmanure compost and a bit of bonemeal. There have been a considerable health improvement in my roses when I started with these amendments. I also use a organic fertilizer with seaweed as foliar feed.
good luck !

Where there are roses, there are lots of blackspot spores. Keeping the leaves clean and disenfecting the soil can only do so much. You'll be much better off planting resistant roses in a sunny spot with good airflow. I wouldn't worry about spores in the soil.

THe best cure for black spot is to choose roses that are resistant to it. Blue Girl is on no one's disease resistant list. Mauve roses as a class are usually the most disease prone color. Check out some antiques that can thrive for years with neglect and thrive under the slightest care. As a whole, modern roses like hybrid teas, floribundas, miniatures, and large flowered climbers are some of the most disease prone roses you can grow, and that's pretty much all you can find available at most big box stores and local nurseries. If you really want to grow roses successfully, it's going to require some research on your part, as well as some effort to obtain the most suitable roses. Check out the Antique Rose Emporium for a start.

A neighbor swears by cornmeal scattered around the roses for warding off blackspot and his roses are beautiful and healthy. Just keep adding more cornmeal as you can no longer see it in the soil anymore. He also puts cornmeal on the weeds in his yard to keep them from germinating.