Big question, is the bottom dark spot soft. If it is hard like wood, replant and water it. The top portion is a desert cactus and needs little watering however the bottom section is a tropical cactus and needs watered more often. I think they use this because most people over water their plants.
If the bottom is soft and is rot, then the only thing you can do is cut if off to good part, let set for a few days and replant in solid perlite and just mist it once in a while until it gets new roots. The very top section can not live on it's own and must stay attached to the green base.
Hope this helps.
I am no expert so I hope you get more replies.
Stush
