This has become one of my favorite dark blue inks. Sei-Boku, along with its dark black sibling, Kiwa-Guro, is a fountain pen-safe pigmented ink. The vast majority of fountain pen inks derive their beautiful hues from dyes rather than pigments, as pigments tend to clog the small channels of fountain pen feeds. And while I’m unsure of if this ink gets its color from only pigment or a mix of pigment and dyes (I’m guess the latter), I can attest that this ink ran through the test pens perfectly fine and without any trouble. And while the manufacturer recommends flushing your pen between fills of this ink, that’s a pretty good ink hygiene habit to get into anyways.
On the page this ink also has a very prominent red/pink sheen, which I’ve found is a common color of sheen for blue inks.
And the one big plus for me with this ink? Here’s what it looks like on cheapo copy paper:
Look at that shading! The best part? It retains its fantastic sheen on copy paper as well! There is only one other ink I can think of that don’t feather and shade heavily and retain sheen: J. Herbin Rouge Hematite 1670. But as for blue inks that share this great behavior? In my experience, only Noodler’s Bad Blue Heron and Rohrer and Klingner’s Salix (an iron gall ink).