Why Are The Leaves On My Plants Going Black?
Leaves are starting to turn black - no powder or anything, just staining
Certified GKH Gardening Expert
That is a severe bacterial infection. There will be no saving it at this point.
When you start over, be sure and use a completely new container, or STERILIZE the one that is there now.
You will need fresh soil, as well. It will be quite difficult to bring the existing soil back to being usable.
Make sure to only water when to top of the soil is completely dry down to about 2 or 3 inches between watering. (That is a general rule, and some plants require different watering schedules)
This article will help: https://www.gardeningknowhow.com/houseplants/hpgen/how-to-water-a-plant.htm