There are two basic types of blueing, chemical blueing which is commonly used by factories on new firearms and by most gunsmiths on most firearms, and rust blueing which gives a much different result, takes much longer with far more labour (ie more expensive) and MUST be used for high quality double barrelled shotguns with the barrels joined by soft solder as chemical blueing will eat the solder and can cause the barrels to fall apart. If your gun is a double, even of fairly modern manufacture, you need to determine for sure how the barrels are joined, any competent gunsmith can make this call with the gun in hand. If you are going with rust blue it will cost you $400 and up, the actual process takes weeks and there are limited smiths in Canada who do this work.