not sure why you cut the growing tip off, if your just re-potting the plant , not a good idea ? or were you attempting to do stem cuttings ? , should only cut growing tips off when doing cuttings only
here is a short vid I made how to do that http://www.youtube.com/watch?v=wF6sQPgLKhc
your cutting are going black mainly be because of the the way your going about the procedure you have taken, , to re-pot a plant should be just a matter of getting as much old mix of the roots gently as possible and putting the whole plant back into the new mix and firming it in and watering it in as well pretty much that's it , unless you intend to take cuttings , then that a different procedure as well , , so what were you going to do as the plant now is in total stress mode , looks like its survival is not good , but acting quickly now may be able to pull it back , just need to know what you want to do , make cuttings or transplant a plant with roots ??