Huh. I never knew that either. Thanks!
I bet it was Japan black. It was quite popular back then as a cheap finish for metal bits. You can scratch it off like paint though. "Sight black" was used to blacken sights to cut glare when target shooting, and still used today by old dinosauers who persist in shooting with iron sights. Most common source of that back then was the soot from the flame of a carbide miner's lamp. Those things were found in every serious target shooters range box, and they didn't use them to see their targets!
We should have a party in Loggah's vault and go down the rows scratching every front sight with a pen knife to see which ones were painted and which ones were blued- to establish a data base for the enrichment of us all. Think he would mind??