Obviously, something has gone awry. For some reason, the stain from the heat treat scale reemerged during tumbling.
I tried attacking it with steel wool, which helped a little bit.
I then tossed the knife back into the tumbler, this time adding a full scoop of borax powder. After about 30 minutes I checked the knife and it was apparent that more drastic measures would have to be taken.
I took the knife to the KMG and hit the chisel side with an A45 Trizact, which easily removed the stain.
I then returned the knife to the tumbler and hoped for the best.
From Knife Making