Good sanitation is extremely important when dealing with this fungus. All leaves that have fallen should be cleaned up and disposed of in the garbage. You need to apply a fungicide containing triforine (Funginex), and you will need to apply every 7-10 days in order to protect the leaves that aren't infected.  Good air circulation also is important.

If you are battling black spot year in and year out you may want to consider growing EarthKind roses that have been tested for black spot resistance.
http://aggie-horticulture.tamu.edu/earthkindroses/

Additional resources:
http://www.extension.umn.edu/distribution/horticulture/DG1163.html

http://aggie-horticulture.tamu.edu/archives/parsons/trees/rose.html

Thanks for the question.