Anything slightly acidic will remove bluing. Hot blue comes off the easiest,,rust blue resists more but will break down and come off.

I use Muriadic Acid (hydrochloric acid), commonly sold in DIY stores for concrete/masonry cleaning and etching. Also swimming pool acid is muriatic.

You only use a couple tablespoons in a gallon of water,,room temp. Don't heat it up to make it work faster.

Dunk the part in it on a wire and leave it. Check it after 5min or so. Should be cleared or at least start to show signs of blue being removed.
Degrease the part first as acid won't cut through grease & oil very well.

I keep a plastic bucket of the stuff as I use it a lot. It's great for removing heat scale from hard soldered pieces too. But the latter takes longer,,couple hours. But no pitting or damage to the metal underneath.
Just don't leave it in the stuff over your weekend vacation get-a-way.

Vinegar works well too,,acetic acid is the working ingredient. Very weak acid but more than enough for the job.
Ketchup will work, kind of,,,has vinegar in it. Messy but if that's all you got!

Any of the Naval Jelly type rust removes work well too, They have phosphoric acid in them.
So do many Cola soft drinks and that's why they'll remove bluing if it's on your fingers and you leave prints on the gun.

Most any of these leave a light gray color on the metal but that shines right back up again with nothing but scotchbrite or even steelwool. No need for heavy polishing unless you need to remove scratches and pitting that were there before.