Cut out that black branch now. Follow it down to where it joins another branch or if it comes from the ground follow it to where it joins the root stalk of your rose and give it a sharp yank to separate it from the mother rose plant. I suspect this cane is a sucker or volunteer from the grafted root stalk and it will try to take over the entire rose plant. These suckers are often less disease resistant and so it is showing the signs of a viral or bacterial disease and needs to be removed. Keep growing, Marianne Binetti