Black Spot on roses is usually found only in areas with low air pollution. In bad cases in my garden, the rose drops almost all its leaves. One thing I'd be disinclined to do is have tomatoes as companion plants for roses. Any gain via the warding off of Black Spot (I have my doubts...) would, for me, be far outweighed by the aesthetic degradation resulting from the juxtaposition of rose bushes and tomato plants. I simply don't see them as belonging together. Assiduous removal of affected rose leaves can help to reduce the infection. Otherwise, you must go inorganic and use a systemic fungicide, or choose roses with at least some resistance, or just not grow roses, which would be a pity. In my experience, the disease doesn't kill the rose bushes and they always seem to try again next year...