I use Casey's bluing paste. I have gotten some really great results with it. This thread contains pictures of an STI LS40 where I touched up holster wear on the frame and slide - http://thefiringline.com/forums/showthread.php?t=508067
Note, this is a flat finish. The paste works good on polished finishes to, but it won't hide imperfections - just makes it black and shiny. Although I haven't experienced any problems, some folks will tell you that even if you get good results, any cold blue will rub off faster than hot bluing.