....lifting of the primer from the pocket[/b], flattening of the primer against the breech face, and deep striker indentation, especially in comparison to shells of the same batch, as evidence of excessive pressure....
Just on speculation, the primer of the blown shell does not appear any more flattened than the sample fired shell next to it. I think the good fired shell is showing a bit of primer flow around the firing pin, maybe a bit warm anyway for the clearance around that firing pin fit up. The lifted primer and the apparent outward bulge on the primer near the striker indentation may have happened as the headspace started to increase as the chamber failed. I do not believe the primer striker indentation area had more strength than the failed barrel, maybe the rim and primer stopped distorting as the pressure dropped when the chamber failed. I would think the plastic hull sealed the entire chamber, from breech to opened crimp, but the plastic failed as the barrel failed in the same direction as the barrel failed, no plastic flowed back into the defect between the rib and barrel. It tore outward, and the plastic hull does not appear to have burned through and thus reaching the steel to burn it.
I still think the dark color is from long term corrosion for two reasons. There is bright metal where the barrel failed away from the rib even though it was exposed to the same conditions as the suspected failure area. And second, it does not appear that the bore cleaner removed the dark coloring as if it were only powder fouling. It's always appeared to me that braze is still bonded to the rib and corrosion ran along the barrel. Only guessing Doc Drew because it is interesting, take care.
