When Krags were built, very few of the parts were 'blued' and the process, (not the color), was called 'browning'.

The parts that come to mind that were rust 'blued' (browned) are the barrel, trigger-guard, some sight parts, the front barrel-band, butt-plate, magazine cut-off lever and some screws.

The extractor was given a 'flame blue'.

The bolt-body, springs, and some rear-sight surfaces were 'in the white'.

The receiver, bolt-collar, safety-lever, magazine-gate, side-plate, and many small parts were casehardened. (In time, the case-colors took on a silvery color).

IIRC - Hydrogen Embrittlement is caused by acids acting on steel and depleting certain elements in the alloy and changing its properties. I believe fire and Naval Jelly can also do this.

I don't think this effect is related to steel being casehardened. It can 'attack' most steel alloys. (Just my perception and I am certainly not a metallurgist).

FWIW - I enjoy the mixture of colors and patina found on the metal of most Krags. Homogeneous 'Bluing' just never looks right to me.