@epyzhyk, unfortunately that looks like a stem rot. Even if there are roots below the rot, the whole stem might eventually rot.
What time zone are you? Not sure how far the rot is. You might have to start over and cut the rotted part until you have just a healthy stem. (Sanitize after each cut.). If the remaining stem is long enough, you might be able to save it. Then you will have to dry and let it callous again before attempting to reroot. Needs to have bottom heat and keep at 6 hours of full sun (not below 50's). Water once and do not water until leaves start to grow, unless the soil is totally dry you might have to mist but do not overwater.
