There are a number of metallurgical situations when oxidation is actually protective (think about the protection aluminum oxide provides to the otherwise highly-reactive aluminum).

Another case is the protective oxidation layer (typically a "mottled grey" not the same as "red" rust) that high-carbon steel knives tend to acquire.

From Gemini AI:

What Patina Does (The Good Stuff)
Protects Against Rust: Patina is a layer of stable, oxidized iron that forms on the steel, preventing further, harmful corrosion (rust) from forming and damaging the blade.

Enhances Edge Retention: High-carbon steel can be hardened significantly, allowing it to hold a razor-sharp edge much longer than stainless steel. The patina helps maintain this performance by protecting the steel from rust, which would compromise the edge.

Creates a Unique Look: Patina creates beautiful, varied colors (blues, grays, purples) that tell the story of the knife's use, making it unique to the owner.

Patina vs. Rust (The Bad Stuff)
Patina: A desirable, protective discoloration that seals the steel.
Rust: Harmful, flaky corrosion that pits the steel, degrades the edge, and can damage the blade's integrity.

How It Relates to the Edge
A well-developed patina means less rust, which means the edge stays cleaner, sharper, and performs better. You'll sharpen less often, as the carbon steel holds its edge longer anyway.

In Summary
Embrace the patina on your high-carbon Japanese knife; it's a sign of a well-used, well-loved tool that's actively protecting its superior edge. Just remember to wipe it dry after each use to keep it from turning into actual rust.

Jeff