I suspect it could be sooty mold (fungal growth). Is there any insects (particularly aphids) on your plant?
The sooty mold generally appears as a black blemish or coating on plant surface. It won't cause any direct damage to plants but may block sunlight and reduce photosynthesis. Sooty mold mainly develops on honey dew (sugary substance) secreted by sap sucking insects like whiteflies, aphids, leaf hoppers, scale insects, mealybugs and psyllids.
Sooty mold can be easily removed by spraying strong jet of water or washing leaves with soap water. And sucking insects can be reduced by spraying neem oil.
Sooty mold on california laurel plant