H&M BlackNitriding process explained: https://www.youtube.com/watch?v=AabpDN0Deo8&feature=emb_logo

From the video; BlackNitriding is a two-step process; 1) surface diffusion of carbon and nitrogen to harden the surface, and, 2) iron oxide formation for corrosion protection.

Another article which provides some info about H&M's BlackNitriding process: https://www.shootingillustrated.com...ros-and-cons-of-nitride-barrel-finishes/

It appears that H&M uses a standard salt-bath based nitrocarburizing process for its first step to caseharden the metal (and, nitrocarburizing is generally acknowledged to be an outstanding metal finishing process), and then, adds the second step of iron oxide formation to enhance the corrosion resistance. Thus, H&M's "BlackNitriding" is apparently an improvement on the standard nitrocarburizing process. There are likely other proprietary tweaks to the process that H&M adds, and doesn't reveal.