Because black spot fungus drops its spores onto the ground around the roses, it's important to pick up fallen leaves, etc. as everybody else has suggested. I've also found that planting groundcover that comes right up to the rose stems but is loose enough to allow stuff to fall through seems to work to reduce blackspot. I have violets planted below/around my roses and those seem not to have as bad blackspot. Roses that I have with bare ground or bare mulch around them seem to suffer more.
