the only cold blue that actually can be used to refinish/restore a gun is oxpho blue. I use it all the time in cases where there is excess welding like a torch cut gun kit. if you want to be a purist go ahead but you gun will look like crap if you park a gun that has welds. no matter what you do even if the welds are not visible to the eye they will be after parking.

I use oxpho blue and it works great. I just restored a bren for someone with it.

I find it even gives the colour of park a bit, its not your average cold blue.

some before and afters











A BAR I did with it





and a very rough Inglis HP, before and afters, notice the serial number in the white now













so depending on your situation, I wouldnt say bluing a gun allways depreciates the value, it has its values and depends why and how you use it as well as the brand you use.

hope this helps