I tend to leave older guns in this condition as they are.
Adds to their charm.
Attempts to partially reblue can be disappointing.
If you keep using an oily rag now and again it shouldn't get worse.
But if you want to have a careful go with very fine wire wool and oil that's a start.
G96 gun blue Creme is the best cold reblue imo.
If you wish to go this far.