This is a carbon steel knife. That means this knife is made of non-stainless steel. There is no or an insufficient amount of chromium and/or nickel to protect this steel against oxidation. Steel such as this will react to moisture and oxygen. That on itself is not a bad thing, you might even argue it’s a great thing. The knife will form a wonderful patina, it will sharpen easily and the edge life will not disappoint.
There are numerous different carbon steel made in Japan. Of which these are most famous and commonly seen in knife making: White steel (Shirogami), Blue steel (Aogami) and SK- high carbon steel. There is also Swedish carbon steel, even vintage Swedish carbon, Tamahagane, and Takefu White but for now we'll focus on these first three steel made by Hitachi Metals Ltd.
SK steel was the first on the market when Japan started mass producing steel with imported iron. SK steels have a relatively low carbon content and higher levels of impurities than the other steels. Knives made with SK steel are often characterised with high toughness due to the lack of carbon, high sharpening ease, relatively good edge retention and quick oxidisation. They will patina fast and react quickly to acidic food but you'll find that they can be surprisingly easy in maintenance once a proper patina is set. Therefore forcing a patina is a popular choice on these knives.
SK steel is a wonderful tool steel that has been very popular for many years and is still being produced today. A prime example of this is the "Kanto Gyuto", Japanese made western style chefs knife. . A almost unchanged design tried and tested over the last couple of decades these knives are still immensely popular nowadays as the workhorse of the kitchen. Now widely available made with stainless steel, back in the day most where made with SK.
SK steels played an vital roll in Japan but soon the demand grew for more refined products with fewer impurities and with performance characteristics more similar to traditional Tamahagane.
The care for a carbon steel knife such as this is not very elaborate, and very comparable to any other knife. Always store your knife clean and dry, no dishwasher, don’t let it air-dry and avoid extremely hard products like frozen foods or bones. With carbon steel you want to dry your knife also during cooking, never leave it moist or wet for too long. Often a quick wipe a dry cloth or towel is more than enough. A carbon steel knife can rust if not taken care of properly. Rust can removed but it is of course something we want to avoid.
Please note that there are many different carbon steels, just like stainless steels, each with its own properties and characteristics. Each should be judged individually.
** EXTRA**
There is no right and wrong between carbon and stainless steel but a fact is that carbon steel is a superior steel for knife making especially when using traditional methods. So what we often find is that carbon steel will have a slightly better treatment, edge retention and sharpening/grind compared to others in the same price category.