Mike, that button may have taken a long time, but it looks to be worth the effort.
Very subtle finish on that one, and I really like it. Yes, I too try cold blue on just about any metal to see what it will do. It worked great on that button.
I'd be proud to use one of those on a bag I made.
One other feature that is important to me. I see that you allowed a large loop opening in the back for thread to pass through. I use heavy linen thread to sew on buttons for security, and many vintage looking buttons are useless to me as I cannot get my needle and thread through the loop. Well done in that one.
God Bless, Marc