Aogami Super Blue Steel
Japanese steels are commonly known by the colour that the label wrapped around the steel happens to be (white, blue, super blue). In this case it's super blue steel, also known as Aogami Super Blue. This is considered to be just about the best mix found for a high carbon steel for knife making. Carbon steels can deliver amazing levels of sharpness and Super Blue is considered to be about as good as it can get. Easy to sharpen and giving a silky smooth cut that we've struggled to top.
The Fujiwara Denka are currently the hardest knives we sell with an HRC of 65:66. They have an exceptionally hard Super Blue Steel cutting edge and are clad in Stainless to make care a little easier.
The Masakage Koishi range have an exceptionally hard Super Blue Steel cutting edge and are laminated in Stainless.
In the quest for the "perfect knife steel" there are always compromises. In the case of Super Blue there are a few to consider. It can chip which is why it is often wrapped in a softer stainless steel to give it more support. Knives made from it are built for their cutting ability not for abuse, you wouldn't go offroading in a Ferrari so ensure you are buying it for the right reasons.
It will discolour and take a patina through use as it oxidises, it does not affect the performance of the knife but not everyone likes this. We see it as the knife taking on a unique character all of its own so it's fair to say we like it! Again this is why it is often laminated in stainless steel to protect the cutting edge.
There are other un-seen compromises too, Super Blue steel is difficulty to work for the blacksmith. This leads to more ruined or unusable blades which pushes up the cost of those blades that are finally finished. This accounts for the increased cost of these blades.
Summing up, it's about as good as carbon steels get, allows for an insanely sharp edge but need some attention to ensure you keep getting the most out of that edge. For a more robust/forgiving carbon steel take a look White Carbon Steel.