I have a duckbill flash hider that's blued.  I'd like to use it for a build, but it should be parked instead.  Is it easier to try to find one that's parked, or can this one be sent somewhere to have it parked?

If you’re talking about a repro then the Brownells ones are still available.. unless you have a friend that can refinish it for free I’d just buy another one

Retro Arms Works.

+1.

It would normally cost more than just buying another one but I already have the stuff here. I use like vinegar or an acid to remove the bluing. Lots of how-tos online. Then I have a bottle of Manganese Phosphate/Parkerizing solution here. https://www.palmettoenterprisesparkerizing.com/product-page/manganes-pint-16oz  (They also sell on eBay)

I heat the solution to 200 degrees in an old coffee pot, drop my small parts in for about 20-30 minutes and done.

I have a few FSB's I removed the flashing and lug from. I have some lower parts I have "aged" too much. Small parts that I dimpled. Bolt releases that need reshaped to 601 style. I have ran across a lot of parts that needed to be manganese phosphated / parkerized so I keep the chemicals on hand. If you think you will need lots of things manganese phosphated / parkerized in the future, then the price of a bottle would pay for itself.

https://www.brownells.com/tools-cleaning/paint-metal-prep/metal-bluing/zinc-phosphate-parkerizing/?sku=082091016

This makes a gray to gray green color.  Just stripped and did two birdcages that were black.  Don't match my barrel too well since it is almost black,  One is gray the other a gray with a slight tint of green.  Took me a few trys to get it right.