The bbl was blued w/the plug assembled in it?
If it was, it's not uncommon for any oil or grease to creep out of the joint and out onto the surfaces and spoil the blue finish.
Every time you run the part through the boiling water to turn the rust to blue, the hot metal thins out the oil/grease in there and it runs out like penetrating oil.
If that is what happened,,I'd pull the plug ,& degrease everything. Then carefully touch up polish the area were the blue didn't take. You just want to brighten it up a little but keep the polish the same as the rest of the metal.
Then rust blue a couple of coats to those areas alone to build up the finish.
Then one or two coats to the complete part(s) to blend everything in.
You didn't say what type of bluing you used (cold/slow rust or hot/quick rust). But for the first couple of coats to the small areas just to build some color, I'd use hot/quick rust blue.
You can do it fast and control it easily.
Once that's done, go over the area AND the entire piece to even things out with a piece of clean fine scotchbrite (gray or maroon color). This does a lot to blend them together and allow the final coat(s) to match up.
Finally go over the entire bbl and tang w/ a coat or two of what ever you used originally to blue them.
Assembling after all the finishing is done.
