Black stem is a warning sign that should be handled early. If the darkened area spreads or softens, the plant may be suffering from rot or severe stress.
Check the stem, roots, and lower leaves carefully. Remove unhealthy tissue with clean tools, keep the plant dry while wounds callus, and avoid returning it to the same wet or poorly ventilated conditions.
Fast action matters. Succulents can recover strongly when the damaged area is removed in time and the plant is given a cleaner, drier recovery environment.