@bellegallica9a neat, congratulations! Hopefully, you'll get some "normal" blooms so you can easily compare!

Thanks! I had kind of forgotten about it. Then a couple of days ago I noticed a bud and thought again, "That bud looks really dark for that rose." Then when it opened it was still darker than usual. It also didn't have the contrast between the front and back of the petals. That's when it finally dawned on me that it might be a sport. I'll try to root a cutting from that branch.

Let the branch grow out and keep watching it for the variable flower. You want enough growth producing the sport so when you root it, you aren't using the entire mutated growth, so in case it doesn't root, you don't lose it. I'd also suggest marking that cane with a piece of yarn or string so you can be sure where the sport happened so it doesn't accidentally get pruned off or otherwise lost. Good luck! Fingers crossed for you!

Done! Thanks so much for the advice!

You're welcome! I hope it helps!