Mervyn,
I am writing from memory that according to The Parker Story, grades through G had blued triggers, and D and higher is where other finishes came in such as nickel, gold, etc. The bluing was done by heating a file to red hot and setting a trigger on them which transferred the heat and color to the trigger. The closest and easiset method is to Nitre Blue today, although the original process should still work. This previous discussion about Nitre-Bluing should help you.
http://parkerguns.org/forums/showthr...=trigger+guard
If this is inaccurate, I invite corrections.