Question about the "forge black" on Iwasaki Kamisori...

I could have sworn a long time ago, I saw something about how these were forged, etc. but I can't find it anymore.

What I wanted to find out was, is there something that is placed on the blade to get the black coating that remains once the razor is ground and honed? Thanks!

Sometimes it's nothing more than fire scale (think high temperature rust = iron oxides) left on from forging and heat treatment.

What Mike said. I sometimes put gun bluing salts on knives & after you get about 5 coats it looks very dark/black.