That is pretty bizzare.
But speaking of Darne metal finishes, lately, I have seen several lower grade Darnes for sale with particularly vivid case colors on the actions. They appear to be cyanide colors. Is that typical of lesser Darnes?
Post WWII, cyanide case colors on R10 Darnes. They soldiered on with XT steel until the Bruchet era. The higher grades no longer came with case colors because the steel was upgraded to XTC, for all intents, a French version of 4140 chromoly. The hardening is French grey on those, similar to color case hardening, but, the quench takes place in a solution of potassium prussiate, instead of water.
The barrels of magnum and higher grade guns are also XTC, which, is tough to blue.