This is my 240mm Konosuke Fujiyama in Aogami #2 (AKA Blue #2) steel lam (the steel with slightly lesser wear resistance than Aogami Super [AKA Super Blue]). Thus far, it is my favorite knife. The fit and finish is second-to-none, the grind is super even, the geometry is exceptional, the balance is fantastic, and the heat treatment was stellar. The Fujiyama is a bit thicker than their other series such as the HD. I also own a HD in White #2.
As you can see, the knife is heavily patina'd. I used mustard and plastic wrap to put the initial forced patina on it for protection, and then over time the natural patina developed. I like the look, and it gives this otherwise very reactive steel great rust protection. Like on many reactive steels, the patina is always changing.
Kono also makes knives in semi stainless steel, stainless steel, White #1, White #2, and a few limited models in various other exotic steels like ZDP-189. There are a few reasons I wanted Blue #2: the specific blacksmith who makes the Fuji line is very well-known and I wanted his work due to his reputation for excellence. As Blue #2 is in the middle of White #2 and Super Blue, it holds an edge slightly longer than White #2, but like White #2 is very easy to sharpen (IMO much easier than Super Blue). I like this fast and easy touchup greatly given this is a knife I use daily and so time saved on sharpening adds up FAST! Toughness-wise, Blue #2 has very good toughness for this kind of knife and the usage it sees. Finally, like White #2, Blue #2 takes a very aggressive edge and really does not care whether you finish at 600 or 30000. I generally stop sharpening somewhere between 1000 and 5000, but the knife cuts like a monster almost regardless of how high I take it. The knife can push cut all day long, and it can also slice like a box cutter. Getting it to a point of raving sharp is very easy to do as it requires little effort on my behalf, and it stays raving sharp for quite a while. While there are a few really high-end stainless steels that can hold an edge longer, I've never used one nearly as easy to sharpen to such an aggressive edge with such little work that has so little pickiness about how you get there. I think that is the primary reason White #2 and Blue #2 are considered some of the most desirable steels for these kinds of knives (because while they hold an edge well, they are more notable for being super easy to sharpen and taking a really aggressive edge with minimal effort).
As for reactivity, Blue #2 is like Super Blue and White #2. It's a super reactive steel, and it can start the patina in literally seconds to a few minutes from when you cut something acidic if you do not wipe the knife off immediately after usage. As that is not always practical to me, the patina worked out well. Now that the knife has the patina, it's great at resisting corrosion.
As a user, usage of super reactive steels generally requires different action. Mainly, it is important to rinse and especially dry a knife shortly-to-immediately after usage. For me that is easy because I did this with stainless to avoid making one food taste like another or cross-contaminate. Obviously these steels in a dishwasher or drying naturally are really out of the question. I do not find keeping Blue or White steels without a patina practical (and I like the natural look as they add character.) So while you still want to wipe the blade with the patina, the patina allows you to not have to wipe down the blade every few cuts almost obsessively.
My personal advice is if you are thinking about trying a reactive steel that you should go for it. IMO White #2 and Blue #2 steels make cooking more enjoyable by greatly cutting down the time you need to maintain them, giving you more time to cook or eat.
