Certified GKH Gardening Expert
It sounds like you have a little bit of what is called Rose Canker or die back. Sometimes it gets into the canes when pruning the roses or deadheading them if the pruners are a bit dirty. I would recommend wiping down your pruners with some Clorox or Lysol disinfectant wipes, then prune out the blackend ends and seal them with Elmers or some good white glue, but not school glue. This article should also help: https://www.gardeningknowhow.com/ornamental/flowers/roses/pruning-roses.htm
This blackening can also happen if pruners are a bit dull and squeeze the cane or stem a lot before actually cutting it.