Hi Alex,
Is this the Alex we got an animal from a while back?
Anyway, to answer your question, obviously the first thing you need to do is get more humidity in the cage. If they aren't shedding, it's not humid enough. You are going to have to figure out something to add humidity for at least a few hours a day, a misting system maybe? Even though ours are kept in a room open to the outside, we still run a hot and a cool air humidifier in the room for several hours every day.
What we have done in the past when we've had a problem shedder, usually a fresh WC, is get a good size container compared to the animals size, lay a towel that is dripping wet with warm water in the bottom, put the animal in, and lay a towel on top of the animal that is very damp, but not dripping, set the whole thing on top of some kind of heat (a heating pad on low is perfect) and let the animal sit for 15-20 minutes. Then take the animal out and with wet hands try to rub/peel the shed off a little bit at a time, dampening the animal with body temp water as it starts to dry out. You may need a helper depending on size of animal and how agitated it gets. You will probably have to repeat the whole thing several times, maybe even over several days from what you wrote. Just stop once the animal starts to get too agitated. In other words, when you start seeing lots of red liquid running down your hands and arms it's time to stop for the day. LOL
I'm a little worried about the black you mentioned. There could be necrosis underneath the old shed. Be very, very careful when working around that area. If so and if it's over a large area and/or deep, a vet visit is probably in order as the animal may need oral/injectable antibiotics. If it's a small area and not deep, then you can try putting Polysporin or Neosporin on it for a few days and watch it closely.
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Karen Clark
Gems of the Rainforest