I agree, continue to
bathe him and to use the cocoa butter. Does it seem like the darkness is spreading up the tail at all? Does it seem any harder?
How old is he? Are you sure he isn't semi-brumating? Is he eating or moving around at all?
If it really seems like the skin is hard and is quite dark in color as well as you having concerns that the circulation might be affected in the tail, you could put some oil on it, like cooking oil quite heavily and wrap his tail up in a hand towel over night, that will really help to hold the moisture in.