Doing a touch up cold blue on any surface under the wood well not hurt value. It's not a 100% gun to begin with. Just touch up the bare spot. You really don't even need to do that. If it under the wood just smear a very light coat of grease over the bare spot and it well not rust.
Personally I leave the bare spots, part of the rifles personality and history.
These old mil-surps are worth more in the original issued condition then in a refinished condition, that is to most people. I've seen some pay top dollar for a tarted up Piccadilly whore (reblue and refinished wood)