When it has absolutely gotta be a cold blue, then there's only one Cold Blue I would consider and that is Oxpho-Blue from Brownells.
It is the best, hands down!
And, I can say this from experience, not something I read in a book.
I have used this bluing for years and never had a problem. I have done several long rifles, a whole bunch of handguns, and a couple of Side by Sides with really fragile ribs, and it did a great job.
The nicest part of this solution, IMO, is the fact it can even be "polished" during application with 0000 Steel Wool....I have also heard you can apply this stuff with "0" steel wool....it's that good.
Just keep your strokes straight, and none of this round & round stuff that often brings out uneven scratches.
In fact, after bluing, you can even buff the bluing with a large soft wheel and Red Rouge if you keep the speed down under 2,000 RPM.
OXPHO-BLUEŽ | Brownells Uncle Russ...