Looks like Sooty Mould, and that usually occurs in association with a scale infestation. Check the plant thoroughly - examine beneath the leaves and the woody parts, you're looking for white fluffy deposits or shield shaped insects or tiny, yellow nymphs. Spray the plant with a treatment for scale, including under the leaves, and spray till run off. The sooty mould will disappear on its own once you've got rid of the scale, but it will take time. More info regarding scale in the link below, including treatments

https://www.rhs.org.uk/advice/profile?PID=224

Also, scale, mealybugs, and/or aphids on any tree or shrub that overhangs the Pieris may be the source of the honeydew that feeds the sooty mold--so look up, too.

It's Sooty Mould, which feeds on the honeydew produced by scale insects. Most of it should wash off, but you need to treat the source of the problem (ie. the scale insects) to prevent further unsightly damage. Bug Clear Ultra is often recommended.