I have not yet. It’s on my list of things to play around with at some point. Mainly I’ve heard/seen it used to display the pattern on Damascus as demonstrated by that beautiful knife dan308 posted. The high nickel alloys don’t take the color and gives you the contrast. Supposedly the coffee really makes the 1084 or whatever your using dark, even more so than the ferric chloride.
I’ve used vinegar, lemon juice, onions, etc but for my purposes cold blueing works best so far. I’ve made some pretty dark too.
