Sailor SouBoku is the latest nano-pigment ink from Sailor. It is a darker and more muted version of Sei Boku. Where Sei Boku has more vibrance and more noticeable green component to it, Sou Boku is a very somber classic blue-black. I reserve my personal judgement of this ink's color for now, but concede that it does look quite nice on ivory paper. Sei Boku is still my personal preference for its more cheerful hue.
Being a nano-pigment ink, water resistance is fantastic. Drying time is very quick as well.
This ink sheens quite readily, but the sheen is not at all distracting or very "in your face". It's just a subtle hue shift toward a red-black, giving the writing some character. I've added some super sheening splash samples, but you won't see that much sheen in normal writing.