Look for aphids, either from it, or from trees overhead. They exude honeydew, on which forms a sooty mould. If that's the problem, prune the rose to tidy it up, tie in any strong new growth etc. feed it, and sort out the aphid problem, wherever it's coming from. The mould wipes off, and does no harm except when so severe it shuts out light from the leaves on which it has formed.

Thank you. It is possible that it is coming from a Japonica.

I have this one and it is a bobby for black spot. This year I sprayed it with RoseClear early in the season and the parts that were withing reach of the spray were much better. I should have done it again later on but didn't.

Thank you. I will make a note to spray early in the season next year.