There are cold bluing products available from Brownells and Birchwood Casey that you can use to treat spots. It you take your time you can do a good good match job. Your other option is the have the gun re-blued by a gunsmith who has hot bluing tanks. I did an entire rifle with Birchwood Casey cold blue after taking it down to bare metal. I then boiled the barreled action in a trough of water so it would flash dry when I removed it and put on a coat of bluing with a cotton ball, then repeated the process over and over again rubbing lightly with 000 steel wool between coats. It come out black and shiny and it still looks good 40 years later. Pics below were taken a little over a year ago, I did the rifle in 1971. You might be able to use a heat gun or hair drier to heat the spot to be treated.
