As you can see from this fact sheet, Jeyes fluid is a very toxic substance with many many warnings about its danger to life and health. It is closely related to creosote.
It will kill anything which it touches, in any dilution, and does not decay on contact with soil. If you want to kill all living things in your flowerbed - worms, spiders, the lot - in addition to some blackspot spores Jeyes fluid is the way to go.
I grow a lot of roses. I live in a rainy part of the country. The best way to keep blackspot at bay is to pick off affected leaves and burn them, pick up fallen leaves and burn them, keep the ground around the rose bushes hoed and crumbly so that the spores have less chance of boucing up off the soil during a rain storm and making sure that there is good ventilation around the plants.
Jeyes fluid is a foul thing. Personally, I think it should be banned.