Cold blue touch-up kits are acceptable for small spots, or firearms that don't get handled much. But, it will wear over time; and failure to properly seal the touched-up spot will lead to accelerated rust.
On one recent project, I was completely unable to get Birchwood-Casey's Perma-Blue to work on wear spots on the barrel. The metal was so porous, that I couldn't get all the contaminants out. Without a clean surface, cold bluing products won't work at all.
Some people complain of difficulty in getting the cold blue products to match the existing bluing, but it can be done fairly well - with a little practice (and multiple applications).
In my opinion, the cheaper the firearm is, the harder it will be to get good results with a cold blue.