Mike,
I've got a couple of competing thoughts here, so bear with me.
I have had all sorts of black water, fouling, etc etc on my guns with Chambers Oil finish and it wipes right off no problem.
You didn't mention, but I will assume that you tried wiping it off first. Assuming that didn't work, then I must wonder if your finish was not enough, and didn't fill the pores of the wood. Added to that, you mentioned that a light buffing with steel wool cut the stain out.
I have used steel wool on Chamber's finish to cut back the sheen and to even out streaks. The first time I tried it was after too few coats of Chambers Oil and I got down to the stain on sharp edges by mistake (also, my finish was applied lightly and was not deep enough). After learning that lesson, I did not buff the finish until later in the process.
That being said, with enough coats of Chambers Oil, I have several times buffed it lightly with steel wool or a scotch brite pad with no problem.
All the above is to echo Jerry's comment that it shouldn't go through to the stain unless the oil finish and the stain are too thin.
However, I have watched you build rifles and know your skills, so I fully expect that you applied the finish correctly.
Could the water have migrated into the wood (underneath the finish) from inside the tang mortise? How far did the black streak go? If it was a long way, then this is not the answer.
Could the final coats of the finish have somehow separated from the wood, allowing the water to migrate below the finish and into the wood? I'm thinking some sort of contamination around the tang that prevented the layers of finish from adhering to each other. Could the finish have been lifted up at some point when your removed the barrel (unlikely)? I would think that you would have noticed that beforehand, because separated finish layers would cause an opaque look.
Did you use some sort of solvent in the cleaning solution that may have degraded the finish, allowing the black streak to penetrate?
There must be something else going on here, because Chambers Oil is a good finish and I know that you take great care in every step of the build.
I won't comment on how to fix it, because I don't know the best solution!! If somebody tells you, I'd like to hear because I have a few pin holes that have split surfaces from punching the pins out to remove the barrel, (ha ha, goes back to why this all happened doesn't it??) and would like to remove the finish, re-stain, and re-oil....
Cheers,
Norm