I recently lost one of my favorite succulents to black stem rot. This is an infection that can be caused by overwatering. I found this guide from SF Gate that has steps on how to treat it.
Basically, these plants are used to a dry arid climate and they need enough water to sustain themselves, but not so much that they are humid. Anything you can do to simulate their natural environment the better.
Signs of overwatering:
1. puffy leaves (they look bloated)
2. dry/shriveled roots (the plant actually needs less water in this case)
3. blanched looking leaves (the coloring is faded because there is too much moisture)
It’s pretty sad looking! 🙁
It turns out if you find small black spots early you can treat your succulents and save them from turning entirely black.