I also recall a method using cold blue and Clorox but i never tried it. It is said to be rather aggressive.
The cold blue/Clorox method, as described to me by the late Bob Harn, was to pre treat selectively the areas to be pitted with cold blue solution. This is then followed by applying or immersing the part in the Clorox. The Clorox will rapidly react to the pre treated areas, while the reaction to the untreated areas will be much slower, essentially leaving those areas unaffected.
The surface is then rinsed and carded with steel wool or Scotchbrite, followed by the desired final step of bluing, browning, or brass darkening solution…
The advantage of this system being the ability control the extent and the location of the pitting to only the areas desired.
Jeff