Years ago every machine shop had a vat which contained a copper sulfate solution. Parts need to have cut lines, hole centers etc laid out on them were first dipped into this solution & received a light "Flash Plate" of copper. When the lines were then scribed on they were much more visable."Layout Ink" which is either brushed or sprayed on has now replaced the copper sulfate solutions. Early "Touch-Up" blues were of a two part nature, the first being a copper sulfate solution which was swabbed on until a good coat was obtained. The second part containing ammonium sulfide was then painted onto the copper & produced "Black Copper Sulfide". The color by this process was produced only in the copper coating, which was not particularly strongly adhereing & wore away quite easily, thus not generally very satisfactory.
Later "Touch-up" (cold) blues are of a one part process & as I recall have Selenium as their base ingredient. The color they produce is a form of oxide produced in the bbl steel itself & while it may not be as strongly adherent as regular slow rust blue it is much more durable than that produced in the copper plating. There is an odor in applying these blues but I don't really believe it has anything to do with Copper Sulfate.
Note the Copper sulfate ingredient Steve mentioned is in very small quantities for the purpose of seeing results on application & in this quantity does not much impede the rusting of the iron/steel beneath it & is likely for the most part removed in the "Scratching Off". For the most part this is either quoted or "Mis-Quoted" from Angier, I'm not a chemist. It is noted that throughout his book Copper Sulfate is found in the majority of formulas listed "Specifically" as applicable for Twist Bbls.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra