A patina is a stable oxide layer that forms as carbon steel reacts with acids and moisture, and it partly protects the steel underneath. To restore one after polishing, clean the blade, then expose it to a mild acid such as mustard, vinegar or fruit, leave it, then rinse and dry. Patina is not rust.
Patina is a stable dark oxide that sits on the surface and slows further corrosion. Rust is unstable, orange, and keeps eating into the steel. A blade with an even patina is better protected than a bare polished one.
Only non-stainless carbon steels form a true patina. Stainless steels will not, because their passive chromium layer prevents it.
Degrease the blade first, or the patina will be patchy. Then apply a mild acid: yellow mustard dabbed on, vinegar, or simply slicing apples and onions for a week. Leave it, rinse, dry, and oil.
Different acids give different colors and patterns, which is part of the appeal for people who like them.
Wipe and dry after use, and oil occasionally. A patina reduces rust risk but does not remove it, and salt will still attack the steel beneath.
If you would rather not have one, keep the blade dry and oiled and polish spots off as they appear.