One of my favorite things about Iroshizuku is the name that Pilot/Namiki gives each ink. Though only the Japanese name appears on the bottle, most stores also provide the English translation (which I assume is accurate and comes directly from Pilot). Tsuki-Yo translates to “Moonlight.” The ink, when wet, appears as a rich blue-black, but when it dries fades somewhat to a dark teal, and hints of blue-green emerge. On certain papers, and when you are writing with a wet nib, you get some pretty good red sheen.
I've had this ink on the top of my ink wish list for a while, together with one of the green Iroshizuku inks. This ink is the first blue ink where I think: damn, I want that.