It sounds like your problem is related to insect waste. Insects like aphids secrete a sticky sweet waste product called honeydew that can coat leaves. Honeydew is clear but a black sooty mold grows quickly on top, leaving plants with a black/grey coating. The weather will eventually wear it off or you can wash the leaves with soapy water. The advantage to soapy water is that it can smother any aphids that may still be on the plant. You might also consider buying and releasing a small packet of ladybugs to devour the aphids. Ladybugs and beneficial insects are available seasonally at some garden centers, or you can buy them from Rincon-Vitova Insectaries or Gardens Alive.