In "Storm of Steel", Ernst Jünger writes about an incoming gas attack in Monchy in June 1916 : "Vorn waren alle Leute beschäftigt, dem so oft geübten "Verhalten bei Gasangriff" gemäss ihre Gewehre einzufetten, deren Laüfe durch das Chlor volkommen geschwärzt sein".

In English: In the front trench, everyone was busy greasing their rifles, the barrels of which were completely blackened by the chlorine, in accordance with the so-often practiced "behavior in gas attack"

He does not speak about the blueing, but I intrepret it as the chlorine attacking the polished parts including the bore (turning black).