How to get rid of black soot?
Certified GKH Gardening Expert
First, you will have to see what pest is causing this problem. Usually, aphids and scale bugs will cause this. You will have to identify the pest, and get rid of them. After this you can gently wash the plant with a few drops of soap in warm water. After, you will want to spray with clean water to remove the soap residue.
For pest control, these articles will give you ideas on how to make your own, along with many more organic gardening tips: https://www.gardeningknowhow.com/special/organic/