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If they still made it, Ortho Seven rose dust.
But it's no longer made.
So you'll have to get Rose Pride spray, and use that first thing in the morning every morning before full sunlight hits it, or before the morning dew burns off/immediately after the dew is gone.
Rose dust can still be found, but not Ortho's rose dust.
(J&P's always dealt better with Ortho's stuff.)

In the old books, the remedy is to cut off the affected parts.
Not an option usually.
There's rotenone rose dust available at Home Depot, but you can't use it near duck ponds or fish bearing streams.
rotenone will work on the rose, but it will kill fish like mad.
My rose was supposed to be a Blue Girl.
(Originally a J&P patent rose, but J&P don't grow them anymore.)
Unfortunately, it is an odd pink lavendar for some reason.
My J&P Blue Girl from when I was growing up was an honest blue.
But that is probably due to the composition of the soil from my old front yard.
Some insecticidal soaps will also be good against black spot, but offhand I don't remember which ones.