Yes - carbon steels such as Aogami (Blue) and Shirogami (White) react with food, moisture, and acids. Left wet they can spot or rust, and with normal use they develop a grey-blue patina. That patina is expected and is part of owning a carbon-steel knife; it is not damage.

Care is simple once it becomes habit: hand-wash in warm water, never leave the knife soaking or sitting in a wet sink, and dry it thoroughly the moment you finish. Avoid the dishwasher entirely, and wipe the blade between tasks when you are cutting acidic foods like citrus, tomato, or onion.

If the knife will sit unused for a while, wipe a very thin film of food-safe camellia oil (or a neutral mineral oil) onto the blade before storing it. A light surface rust spot lifts easily with a rust eraser or a little Bar Keepers Friend; sustained neglect is the only thing to really avoid.

Carbon steel rewards this small amount of attention with a keener edge and easier sharpening than most stainless. If you would rather not think about maintenance at all, ask us about stainless or stainless-clad options instead.

Aogami Blue steel, specifically Aogami Super (Blue Super), is a high-carbon Japanese knife steel made by Hitachi that chefs value for its ability to take an extremely sharp edge and hold it longer than most other carbon steels, thanks to a composition with 1.45% carbon, 2.25% tungsten, and added vanadium.

The "Blue" in Aogami refers to the color of the paper the steel is wrapped in at the mill, not the steel itself. Aogami Super is an upgraded version of Blue Steel No. 1, with higher carbon and tungsten content plus vanadium, which refines the grain structure. This lets the steel be hardened to HRC 62–65 without becoming overly brittle, giving you a blade that can be ground very thin for aggressive cutting performance while still resisting chipping in normal use. The trade-off is that, like all high-carbon steels, it is not stainless—it will rust if left wet or acidic, so you need to wipe it down and keep it dry.

Aogami carbon steel rusts because it lacks the chromium that makes stainless steel rust-resistant, so the only way to keep it from rusting is to never let moisture sit on the blade — wipe it dry immediately after use and store it in a dry environment.

Aogami Super (Blue Super) is a high-carbon steel (1.45% carbon) with no chromium content (only 0.4%), meaning it will rust if left wet. After every use, wash the blade with mild soap and warm water, then dry it thoroughly with a clean towel — do not air-dry. For storage, keep the knife in a dry area, ideally in a saya (wooden sheath) or on a magnetic rack away from humidity. A light coating of food-grade mineral oil or camellia oil on the blade before storage adds an extra barrier against moisture.

Building a patina — a thin, stable oxide layer — is the traditional way to protect the steel while adding character. Cutting warm protein, especially meat with blood (like fish or steak), will encourage a nice, even patina over time. Many enthusiasts treat patina development as a form of art; you can see examples and techniques in the Kitchen Knife Forums patina thread at https://www.kitchenknifeforums.com/threads/my-favorite-color-is-blue-a-patina-thread.299. Avoid cutting acidic foods (lemons, tomatoes) on a new blade until a patina has formed, as acid can cause uneven or aggressive rust spots.