Oooops,so sorry....looked like it was blued.
When going to our jobber,nut bolt joint buying fasteners,sort of just rolls of the tongue...."black"(as in,oxide).When throwing the note tab up on the counter,referencing "finish".
Birchwood Casey has a cold blue bottle that basically provides the same.Machine part,hit it on the ____ op (open to further discussion) ,clean it,dip it,voila....black,blue,whatever.
Nice way to finish a part,nothing more....nothing less.