Why is the elatior begonia flowers turning black?

Make sure your tuberous begonia is in the correct environment. Consistently warm temperature with partial shade. Preferably morning sunlight only.

Powdery mildew can cause leaves and flowers to turn black. Remove the affected leaves. Cool nights and hot humid days will cause PM among other things. Make sure your soil is dry before watering. Begonias will rot if watered too much and water at the soil level, not overhead.

Several products found at your local garden center can help you to rid the PM and prevent it.