Answer: Cleaning pruners between cuts and/or at least between plants can help prevent spreading problems from one part of a plant to another or from one plant to another, yes. The most commonly recommended solution is to dip it in or spray on a solution of one part bleach and nine parts water, or to use rubbing alcohol. This is somewhat corrosive and hard on the tool.
Black spot however is so pervasive that I don't think cleaning the pruners alone would stop the spread of it. Black spot is a fungal problem and spreads from spores on the plants, on the soil or mulch, or blows over from nearby plants. If you are growing roses that are susceptible to it, you garden in an area where it is prevalent, and weather conditions are right, then blackspot will happen. If you regularly have trouble with black spot probably the best solution would be to grow roses that are especially resistant to it.
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