[DonVoigt]

"How was the salt bluing of these parts kept matching with the rest of the parts of the Luger that were not tempered?"

The minimal heat required for strawing would not/does not affect the color achieved in hot bluing.

The mfg. and heat treatment of many luger parts are detailed in the Mauser luger book, you can maybe find a more complete answer there as to the heat treatment if any by Mauser of these small parts.