The Leaves On My Pier Is Are Covered In What Looks Like Black Powder.

Question

It’s in a large pot since 2012.

Answer 1

It could be sooty mold. Inspect your plant for insects. The insects secrete honeydew which causes the sooty mold.

Here is more:

https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm

https://www.gardeningknowhow.com/ornamental/shrubs/japanese-andromeda/grow-japanese-andromeda.htm