Jeyes fluid is a very nasty substance produced as a by-product of the coal industry. Besides sorting out the odd bit of blackspot it will also kill all the good bacteria and living creatures in the soil stone dead. You will notice that tins of Jeyes fluid tell you to avoid contact with the stuff. That's because it does us no good either. It is extremely toxic to all living things.
Blackspot is an air-borne fungus in the same way that mildew is. It flies around all the time in the air and is constantly landing on the soil. When the rain falls, water droplets bounce up from the earth and land on the rose stems and leaves. The fungus gets onto the plants by being splashed up there.
The best way to ensure that you don't suffer from blackspot is to do the following things:
1 Pull off and burn or bin all affected leaves on the plants. Now.
2 Pick up all diseased leaves from the ground and burn or bin them too.
3 Try to make sure that any pruning you do is on a dry day and, if possible, is above the "bounce" level of a raindrop. If the leaves and stems are pruned when they are dry, the cuts will have time to harden and dry off and resist the rain and the fungus.
4 Keep doing all of the above for ever......