How do I get rid of a black spot on my roses?

Since it's a fungus, it will live in the damp soil. Here's a link with ways to treat it:

https://www.todayshomeowner.com/how-to-control-black-spot-fungus-disease-on-roses/

How to: Treat Black Spot on Roses (A Complete Step by Step Guide ...https://www.youtube.com/watch?v=G1K2LYuOnGA

It does not live in the soil but in the infected plant matter that is in the soil so you need to remove all fallen leaves/plant matter from soil and burn them or discard bagged well in trash. Removing all infected plant leaves from plants cut them off & treat the same. Then spray remaining plants to prevent further infection. cut back on watering and never water with sprinklers or from hose onto top of plants this further washes all spores onto more plants. just water directly onto soil around base of plant. Check to see if surrounding trees shrubs are blocking sunlight. Bright sun and good air circulation are essential for getting rid of black leaf spot. For killing spores do this------

There are several good fungicides on the market, several of which claim to be organic. They come in handy bottle sprayers, but if your garden is large, you might want to buy it as a concentrate to mix in your tank sprayer. Neem oil is another alternative for treating black leaf spot. It’s an oil pressed from an evergreen tree. It’s all natural and has shown some remarkable results as an effective garden fungicide. For those of you who prefer Grandma’s solutions to garden problems, try this: Mix one heaping tablespoon of bicarbonate of soda (baking soda) into a gallon of water for your sprayer. Add a dash of horticultural oil or horticultural soap(you can use Dawn dish soap) and Voila! You have a method of treating black leaf spot that works by changing the pH on the leaf surface to one the fungus can’t survive. The oil or soap makes the solution stick and the cost is around four cents a gallon.

Lynn has given you some really good advice so there is no need to add more.

Good advice from Lynn except, sometimes you have to dig rosesup. I had this happen to me and I was told to clean roses roots . I dug up roses put them in soda and dawn water while cleanedup area where they were. I took out 4 inches of soil and made sure area was clean . The put in new soil and replanted roses. The mulch I used was what caused my problem. Be very careful of the mulch you use. I had help with getting all this done. Be careful of leaves and other like matter collecting in rose beds.

roses need to be where they receive direct sunlight most the day to prevent fungus. Be sure they have the sunlight, water only from the base and keep all dead organic matter removed. Also, I cut off the leaves/stems with the black fungus and bag them to the trash can to prevent further infestation. Hope this helps, lots of good info already but wanted to add this. Hope it helps.