When hot bluing bbl'd actions or even revolver frames with their bbl attached, there's always the chance that any lube placed on the threads of either part during assembly will liquify and weep out of the joint betw the two.
That'll spoil the bluing right at the joint and if excessive, can run down the parts and cause even further spread of bluing damage.
No bluing at all, streaks, discoloration, ect.
You can have this same problem in Rust Bluing as well.

The other problem with hot salt bluing the bbl'd assemblys as one piece is that they are not a water tight fit in many instances.
During the bluing process, the hot salt soln makes it way into the threaded joint. Most times that salt soln can be washed & rinsed away in the post bluing rinse.
Sometimes not. Sometimes you think you have it all out and there's still some in there. There's no way of seeing it, it looks clear from the outside.

What trips you up is what appears later on. It may be a few days to a few weeks later.
A white bloom growth starts to appear around the joint. That's the dried bluing salts still trapped in the joint (or any parts you have blued).
The bluing salts attract water from the air and start to 'grow'. That's the white growth you see.
It'll keep growing and crawling onto the outside surfaces of the parts. Since it's wet and alkali,,it rusts and pits the steel it's on. The bluing is gone underneath.

If you've been around to shops and gunshows looking at enough reblued guns, especially bolt rifles,,you've probably noticed the problem on more than a few. Usually the white stuff is brushed off and oiled for a better appearance just before sale time. But the spoiled blue and the rust and perhaps pitting around the joint is hard to cover.
M98 Mausers are very prone to it as the shoulder of the bbl does not fit tightly to the front of the action. That allows the soln to get in there very easily. Tough to rinse completely out.

Oiling the stuff won't 'kill' it. It'll just grow back.
Plain boiling water will disolve it and can be rinsed away. That was supposed to happen in the original rinse after bluing, but didn't.

Sometimes a re-rinse can remove it for you, sometimes not.
Usually the finish is damaged and metal pitted already once it's growing on the outside of the parts.
Then it's remove the bbl time. Flush the stuff off.
Repolish and re-blue as separate pieces and reassemble.

The last part is the safest way to avoid the problem in the first place.

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You're right,,no plugs in the bbl when Hot Salt Bluing.
Totally un-needed and dangerous if done.

Also don't hot salt blue any bbl's or parts with soft soldered (lead) sights, ribs, sling swivel bases, ect attached.
The soln will degrade the lead component of any Lead alloy soft solder. The soldered joint will fail. It may take a few weeks or a few months, but it will break down and fail.
(Ithaca hot-blued the very last run of NID SxS shotgun bbl's in 20 & 16ga's that they made.
Another post war idea of doing things faster.
Then they started to see the soft soldered rib joints fail.
The entire lot of bbl assemblies were scrapped. The guns never re-bbl'd)

Since the soln attacks the lead in soft solders,,the newer (No Lead) Tin/Silver alloy soft solders are supposed to be safe to go through Hot salt Blueing.
I still don't trust them too, but I guess science is not on my side!

Another 'trick' to be able to hot blue soft soldered (lead alloy) joined parts was to add a small amt of potassium cyanide to the bluing soln.
The cyanide salts retards the action of the bluing soln on the lead in the solder.
I've seen that done. I did work for a 'smith that used to do that. He mixed his own salts too. Very proud of himself as a self taught chemist.
I stayed clear of the Bluing Room when picking up and delivering work after that.

I Rust Blue anyway.