If the solder is soft, then you have the right idea, rust blue. Don't pay any attention to the color of the existing blue. You might take a look at Laurel Mountain Forge Barrel Brown and Degreaser( Brownells). It is much easier to use than the traditional rust blues since it contains a detergent.
You can bare hand the parts when applying the solution and during carding as the detergent cuts right through fingerprints and skin oil.
I know it says "Barrel Brown", but it does a beautiful rust blue when used in the traditional rust blue process.
RWO
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