jamese, yes, it is found occasionally, comes from the harder parts not taking the salt bluing correctly due to hardness, time of immersion in the salts bath, temperature of the salt bath, etc. Here is a picture of a Luger I used to own. It was not only plum on the receiver but other parts as well. Mine was a 1938 S/42 next to a 1939 Mauser you can see the color difference. I find these an interesting historical "mistake" by the bluer. It is very common to see EJECTORS and EXTRACTORS of Mauser Luger made after 1937 this color.
CLICK HERE for larger image