The most effective blackspot fungicide in my garden is tebuconazole (sold under the Bayer label) followed closely by propiconazole ( found in Infuse, Banner Maxx and some other products. It is my understanding from what I have read that these two are generally considered to be the most effective nationwide. Both operate by destroying the blackspot spore during the act of germination and also by destroying the adult blackspot fungus, itself. Rosemania (see link, below) claims that the fungicide Mancozeb (found in Pentathlon) is the "only chemical that will actually kill blackspot spores". This is clearly not true in general but it might be true with reference to propiconazole and tebuconazole. Mancozeb alternated with propiconazole or tebuconazole has been recommended as an efficient two-pronged control but: 1)I think propiconazole and tebuconazole are effective enough by themselves and 2) Mancozeb is possibly the least safe of modern fungicides and it's banned in the state of New York.

Here is a link that might be useful: Mancozeb (Pentathlon)

To prevent, you must begin spraying at leafout in spring and weekly or as often as the label states until the first hard frost in fall. There are no once sprays but some products will kill established blackspot, even though the spots will remain. Bayer's Advanced Disease Control for Roses, Flowers, and shrubs is effective if used as directed. Not so much the All in One, 2 in 1, or 3 in 1 systemic products.
As an alternative, buy disease resistant shrub rose varieties. I have 400 roses and don't spray, haven't for 10 years. While I'll get some blackspot when disease pressure is high, for the most part I get little. What I do get, I ignore and don't fret over. I walk quickly past any blackspot.
Unless you exhibit or have a perfect leaf phobia, a little blackspot can be lived with.

Another product that has been discussed in the past was "Messenger", a harpin protein based product. The product has been purchased by another company, Plant Health Care, Inc. (PHC). PHC has a harpin protein product called "EMPLOY". These products are said to help the plant resist disease, including blackspot. My experience with Messenger wasn't positive, I still needed the synthetic fungicide to keep the plants clean and it was expensive.

As mike said, the Bayer tebuconazole-only spray product is proven to kill the blackspot organism in the leaf and will save the leaf if the local infection is not too far advanced--that is, within a few days of small spots becoming visible. Most people here who use it apply it every two weeks except in very hot or dry weather.

I tried just Wilt-Pruf and it appeared to stop the blackspot from spreading. The spots remained but did not grow the rest of the summer.

The following link described a Wilt-Pruf, Rose Flora mixture. The Rose Flora (a soil based bacterium) was chosen because it killed blackspot in laboratory tests.

Here is a link that might be useful: Bacterium Spray Program for Controlling Black Spot.

I just bought something called Actinovate at my conference this weekend. Going to give it a try. I'll leet you know what I think later in the season.

I've used Messenger for years. It makes for nice healthy growth but I wouldn't say it prevented any black spot.

Here is a link that might be useful: Actinovate