If you notice a succulent’s root/stem becoming increasingly dark in color and/or becoming soft, your plant may be developing root rot. If root rot has begun to develop, in addition to becoming darker in color, the root/stem will become squishy (and will even develop a unique smell). If this occurs, take your succulent off of the soil/potting mix and skip a watering or two.
In order to further prevent the spread of root rot, you may want to remove the affected leaves at the bottom of the plant, if any, and cut the stem above where any active root rot exists.
At this point, the best course of action is to step back and let nature take its course, but don’t replant you succulent immediately. Remember to always let any freshly-trimmed plants callous over before repotting or watering, regardless of the situation. If all goes well, you’ll get to see your succulent surgery skills pay off when new roots begin to form, your succulent begins to grow once again, and the root rot is a thing of the past!