One possible reason for black leaves on azaleas is a lack of iron in the soil.

Why Are My Azalea Leaves Turning Black? (And How to Fix It)

Azaleas are one of the most popular shrubs in the landscape, prized for their beautiful flowers that bloom in spring. But sometimes, azalea leaves will start to turn black, and this can be very disheartening for gardeners. There are a few reasons why this might happen, and luckily, there are also a few things that can be done to fix it.