Concrete 6 yrs old, sealed once 6 yrs ago. Animal had chewed up a walnut on the concrete and in the rain it became stained. It had been there two weeks when I tried a strong solution of Oxyclean and scrubbed, waited 15 min, scrubbed, etc for 4 times. It did have some effect but did not remove the stain completely. It was going to take a LONG time. I then rinsed the surface and sprinkled granular chlorine that I use in the pool on the surface, moisted it with a mist, scrubbed for 1 minute and the stain was completely gone. Note: Granular Chlorine is a strong oxidizing agent and is very corrosive.

Thanks James for the helpful tips regarding black walnut stains on concrete! Sounds like you found a successful recipe to eliminate this small, but annoying problem. A power washer with adjusted pressure may also be helpful too!

Thanks for the positive feedback and suggestions!