That's normal, patches will come out black forever...30-50 passes, changing the patch and adding more JB often will do the job of polishing the bore before shooting it. I have JB'd or Flitzed all of my rifles, CZ452 17hmr included, before I shoot them, it's SOP. On the 17hmr and 204 Ruger, I use a Patchworm, I don't like the rod flex of the thin rods.
Tim