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.

Dry it as soon as you finish, keep it somewhere dry, and let a patina build. Do that and aogami (Blue) steel will hardly ever rust.

Aogami #1, #2 and Super all have less than half a percent of chromium, nowhere near what a stainless steel needs, so water left sitting on the blade will spot it.

The routine:

• Wipe the blade as you go, especially after onion, citrus or tomato.
• When you finish, wash it in warm water with a little soap and dry it straight away with a towel. Don't let it air dry or leave it in the sink or the dish rack, and never put it in the dishwasher.
• Store it dry: on a magnetic strip, in a saya or in a block in a dry spot. A damp drawer is the worst place for it.
• If it's going to sit unused for a while, wipe on a thin film of camellia or mineral oil.
• Let a patina form. The grey, blue and brown colours you get from cutting meat and vegetables are a stable oxide layer, and it slows rust down. Orange or red specks are rust.

If you do find rust, lift light orange spots with a rust eraser and a little water, then dry and oil the blade. We stock the CKK and Naniwa rust erasers. Rust on the edge itself comes off the next time you sharpen. Deep pitting is a job for us, so bring the knife in.

On a stainless-clad knife only the thin line of aogami along the edge can rust; the cladding stays clean. A carbon-clad or honyaki blade can rust over its whole face and needs the same care all over.