Blueing can be removed pretty easy with chemicals like Birchwood Casey's "rust and blue remover", but it sounds like you are are talking about parkerizing which is more difficult to remove. Abrasive methods like blasting work best. You can use fine aluminum oxide if you are going to blue it then polish the parts if you want.
There is a chemical method, but I have not tried it. From their website; IOSSO QUICKSTRIP is a revolutionary product designed to remove Parkerizing and blueing within minutes without etching. Just immerse parts into Quickstrip solution and let soak for several minutes. Watch closely until completely stripped, then remove, and rinse thoroughly with water. Eliminates the need for bead blasting. Solution is re-useable. Quickstrip does not contain a rust inhibitor. Easy to use – little odor! Environmentally friendly. http://www.iosso.com/MivaStore/merchant ... unCleaning
Also Brownell's sells a product called "steel white". You dilute it by 50%. I've read it works well to take off Parkerizing and blueing. Just don't mix it too strong, or leave parts in it longer than necessary.
Once you get the park off you can then have it reblued. You can use a cold blue product, but it's not as durable as hot bluing. A gun like that IMO deserves hot bluing.