Turn Rust To Blue
If your bluing is actually black, you could try polishing the rust until it is very smooth and treating the rusty spot with lime-sulphur solution. The sulphides react with the rust to form black iron sulphide. This is an old bluing method, but it still works. This way, you don't have to remove all the rust and leave a pit.
Lime-sulphur spray is used to control pests on dormant fruit trees and the like. If you can't find any in garden-supply places, make your own: dissolve a pinch of powdered sulphur and CaO, lime, in a cup of boiling water. It takes some time as both items are nearly insoluble in water. They have to react chemically with water to dissolve. The solution is clear red colour.
This bluing method is also useful for bluing small items made of iron/steel. You could try it out before doing the barrel. Shine the small items up and expose them to salt water and air for a bit until they are rusty, polish the rust and dip them in the solution. You can also get iron/steel to rust quickly by making the item the positive terminal in an electrolytic cell. Oxygen is released all over the surface and rust happens. Of course, all oil or other impurities should be removed to get a uniform coating. The process should not damage existing bluing if it was done by a similar process, but test an inconspicuous spot first to be sure.
Lime-sulphur solution has a short shelf-life. It decomposes back into sulphur and lime-water after a while, so it is best made fresh for each job.