Well then, from the internet I deduce that these items were case hardened and most likely given additional black oxide coating. I suspect many may have been redone in the 1920's because of improvements to the silvery coating during that era. Hence, the silvery Krag items we have today were not likely ever blued in appearance like we see today. All I know is that I sure in heck would not want a sparkly rifle on the battlefield! Mostly from Wikipedia:

A possible explanation of the silvery finish: Case hardening is the process of hardening the surface of a metal, often a low carbon steel, by infusing elements into the material's surface, forming a thin layer of a harder alloy. Firearms were a common item case hardened in the past, as they required precision machining best done on low carbon alloys, yet needed the hardness and wear resistance of a higher carbon alloy. Parts that are subject to high pressures and sharp impacts are still commonly case hardened. Case hardening involves packing the low-carbon iron within a substance high in carbon, then heating this pack to encourage carbon migration into the surface of the iron. This forms a thin surface layer of higher carbon steel, with the carbon content gradually decreasing deeper from the surface. The resulting product combines much of the toughness of a low-carbon steel core, with the hardness and wear resistance of the outer high-carbon steel. The resulting case hardened part may show distinct surface discoloration.

A possible explanation of the satin black color: Certain case hardening compounds also left a residual oxide surface. This oxide surface works similar to bluing, providing a degree of corrosion resistance, as well as an attractive finish. Additionally, a second finishing step may have added a black oxide coating similar to what is often seen on drill bits which would have worn off fairly quickly however the metal would be more protected until the piece was put into service with regular oiling.