Black marks on a Pieris can be an indication you have Sooty Mould. Sap-sucking pests (aphids, whiteflies, scale insects, mealybugs) that don’t usually kill a shrub but the sooty coating will prevent photosynthesis.
How to deal with Sooty Mould?
Wipe the effected leaves with lukewarm water but change the water and clean the cloth regularly. You may need to repeat this a few days later if needed.