WWII Luger ... Re-blueing is normally done on severely damaged Lugers where chemical reactions, rust, deep pitting Etc. have occurred.
Our resident restoration expert Thor has come to the rescue of many a gun.
https://members.rennlist.com/lugerman/
And G.T. as well ...
http://forum.lugerforum.com/member.php?u=20
Both are able to help anyone with any Luger issue ... We are so lucky