I personally abhor the use of cold blueing, but - there are times it is necessary.
My favourite over the years has gravitated to Birchwood Casey's Paste Gun Blue.
This is more of a cream, than a paste.
I find it give the best most even "blue/black" I've seen. Multiple dags of it rubbed into the
surface, refreshed, over and over until it's perfect, then run under cold water, then wiped
dry, then WD40'd. Done.
Of course if you were going to rub it back to something grayish, that's your forte'.
i've always bought mine form the local hardware store which carries very few supplies/
