This may not be the way to do it, but works
Clean off all the oil with a decreaser like brake cleaner (product with hydrocarbon solvents) would do an excellent nice job.
Number of postings to various on-line gun forums where people reported doing this. The one that that stuck out was from a guy who discovered that vinegar removed bluing when the Taco Bell hot sauce dripped onto his gun because he was going to the range and carrying his gun and some tacos in the same paper bag. When he got to the range and discovered the mess, he wiped off the hot sauce and saw that the bluing came with it as Taco’s Bell sauce lists vinegar as a primary ingredient.
Then pour in enough vinegar to completely submerge all the parts and wait.
The bluing started to come off after just about 15 minutes or so. The steel will turned orange-brown after maybe 20 minutes. Not entirely you will still see some patches of black.
This is what happens when the acetic acid in the vinegar is converting the very thin outer layer of black magnetite to the orange iron oxide of rust.
Remove each part every 20 minutes or so and wipe it down with a paper towel to remove the lifted oxidation. Then place it back in the vinegar bath.
The orange-brown oxide starts to come off with handling.
After two hours or so this should bare metal everywhere. Remove the parts, pour out the vinegar, and place the parts back in the dish. Then place the dish under a slowly pouring faucet for several minutes to flush the vinegar.
Follow that with further careful flushing of each piece and hand drying.
You can heat an oven to 250°F or 120°C and place the parts inside for about 20 minutes on a pan lined with aluminum foil.
The heat will dry the parts and leave some light orange rust deposits.