Virtually all fungicides will be prophylactic (preventative) rather than curative but any containing maneb as the active ingredient (Mancozeb or Fore) may work curatively.

I would still be inclined to remove as much of the disease affected foliage as possible before spraying every 3-5 days for a period of a month or so.

And follow all other cultural suggestions to reduce or eliminate the conditions under which black spot prefers to grow.

The black spot colonies currently on the leaves will not disappear no matter what you or I do. Those black spot infected leaves are doomed. Your only approach is to spray a fungicide, I use the Bayer fungicide, to prevent the spread to uninfected leaves.

There are two solutions to black spot infection:

1. Start spraying susceptible varieties preventatively before it gets a stronghold, and spray on a regular schedule to keep the prevention effective.

2. Grow only black spot free/resistant roses which do not need spraying.

Moses

What I was told years ago by the knowledgeable employees at my locally-owned garden center, confirmed by things I have read thru the years, is to use Mancozeb if roses are already infected. I seem to recall that it will damage flowers, and that you need to spray the entire plant, as it is not a systemic treatment. Hah hah, as if it's possible to cover every leaf top and bottom. I agree with GardenGal about pulling off all infected leaves, taking care that your black-spotty hands (and the leaves you are removing) don't touch uninfected leaves. It's an unpleasant task, in more ways than one. I line a 10-quart metal pail with one or sometimes two plastic grocery bags, collect the diseased material, and tie off the bags and put them in the garbage. (Don't leave the uncovered pail of diseased material sitting around where the breeze can catch the spores.) Good luck. Black spot is my bane.