'flyer92' - That is a correct (NOS) model 1898 Krag bolt. It is a good example of how pretty Krag parts were, when they left Springfield Armory 'Brand New'. (I have an identical looking bolt. They are real).

If I understand correctly, the Krag bolt was hardened with potassium cyanide. It is likely after the 'heat treat', the bolt would have a color.

The 'Standard Finish' applied to the bolt at the end of production was: "Polished".

IMHO - The blackened areas on your bolt are simply the 'low spots' where the color from hardening was protected. It would ultimately wear off from use.