There is copper sulfate (or some other copper compound,,I'm no chemist) in L/M.
You can get the copper itself to plate out onto the metal while applying the solution if you're not careful.
It usually occures on the first coating,,maybe the second. I can't recall ever having a problem later than that.

The easiest way to have it occur is to have the bbls a bit warm.
Applying the soln to warm steel will result in patches of copper wash on the steel.

Another is to go over a spot with the swab moist with the L/M in an attempt to 'give it a better coating'.
Doing that, regardless of the room temp will almost always result in a copper wash/plate on the steel,,though not as distinct as the first.

Sometimes dispite my best efforts, an area will just plate out for what appears to be no reason at all. Usually near the breech (thicker area).
I get rid of it with the scotch-brite trick after the boiling and go on.

When the oily water leaks back out from inside the ribs and covers your work, it can do so while it's still in the tank.
Some oil gets left on the steel (the rest floats up and covers the water w/a slick.
The conversion from red to black oxide doesn't complete. You get the reddish color seen.
It'll card off and only a faint blemish in the blue results,,lighter than the surroundings. But it's still contaminated and will continue to be from the same area in the rib till it's clean.

You can almost get the area to match after several coats. But oiling & wiping it down afterwards will result in a thin area right there as the bluing doesn't 'take' due to the original contaminated surface.

You can contaminate your carding brush(s) going over spots like that too.
Then you are just transfering it all over the steel and back into the water. Even a clean tank become instantly contaminated.

Gotta' clean that out of the ribs.
There's all types of oil and whatever that's been sprayed and pumped onto and into the ribs and void over the life of these old doubles.
Some must think the weep hole was labled 'oil' for all the stuff in there. How some of them have gotten wheel bearing grease pumped into the void I don't know.
A syringe perhaps. Those were probably to quiet the rolling solder BBs.
Have fun!