Very very dark black unlike some. Its also not dry at all so the ink quality is amazing. The smudge resistance is also very good! BUY IT NOW!!

UPDATE: NOT waterproof, water resistant. If you want to use this for art and go back over it, this ink it will blur and smear. (last picture: I let ink dry overnight and added a drop of water). I'm not knocking a star off, though, because it's only advertised as water resistant; I'd seen some ink reviews that said waterproof.The first picture is on Tomoe River to show the ink at its absolute best (within the confines of my abilities, anyway), then in the second it shows that the ink is nearly as good on Southworth parchment. I used a gillott 303 nib on a dip pen. Beautiful, well-behaved ink, and it's waterproof, so it's excellent for addressing envelopes.

I have a small collection of black inks and I wanted to try this one to see if it would flow a little drier than my other more lubricated inks, since I have a pen that is a gusher. It does exactly that, it is drier but it is also gorgeous and every so slightly shimmery under direct light. It does smell a little weird, like maybe medical supplies idk, but it doesn't bother me. I can recognize it by smell now in my different pens.

I love the appearance of sheening inks, but I always hate using them because they are so vulnerable to water that even the slightest contact with skin will leave a smeared mess, no matter how long its dried. Enter Sailor Sei Boku. It exhibits a subtle red sheen on papers like Tomoe River and Midori. It's not the strongest sheen, but unlike all other sheening inks I've used, it's also extremely water resistant, and pretty much immune to smearing. Since I've purchased this ink, I've barely used anything else, and I've almost used the entire 50ml bottle already. It's the perfect combination of an interesting color, while also being practical.

Suitable for almost all fountain pen. Smooth feeling, satisfactory darkness, never block pen.

I hesitated for quite a while before buying this - debating between this (13-2002-242, Seiboku) and the Souboku ink. While I don't have the latter now, deciding after reading many reviews and articles, to try Seiboku (this ink) I'm so very glad that I did! I LOVE the color. Often described as a blue-black, it decidedly leans to the blue, with a little green (teal, some say) but it's just "luxurious!" YMMV, but if you're debating, maybe seeing if you can find a sample to try it. One of these days I want to try a sample of its cousin Souboku, but I just love this one so much I'm not honestly sure of the need for anything else. My other favorite is Platinum Carbon Black (another pigment ink). I call them "security" inks because the pigments (nano particles) truly binds to the paper so that it's almost impossible to remove it (search "pigment vs. dye ink"). They are "permanent" without being long-term deleterious to the paper, as iron gall (at least the "old fashioned" iron galls) could be (at least over the decades and honestly no one will be reading anything I wrote decades from now!). And I find that this flows so much more nicely than the iron gall inks (e.g. R&K) that I have - they're really nice, but aren't as enduring - I've tested them as described below and they are NOT "permanent" in certain solvents as the pigment inks are). If you look up "check washing" you'll see what I'm talking about. There are articles and videos demonstrating it. I took samples of Carbon Black and Seiboku on standard (nothing fancy at all) writing paper, let them dry a day, and soaked them in: 1. Water, 2. Isopropyl alcohol (91%), 3. hydrogen peroxide, 4. acetone - the apparently "classic" ink washer - nail polish remover and 5. household BLEACH. Let them all sit for 3 days soaking. At the end, Carbon and Seiboku were TOTALLY legible in EVERY ONE of these liquids. Indeed, the bleach actually DESTROYED THE PAPER (it fell apart as I removed it from the container, but the INK REMAINED!). I also tested ballpoint inks, iron gall and dye inks in this experiment, some of them literally disappeared with soaking, but not these two! So anything you want to STAY on paper likely WILL with either of these two (Uni-ball pigment gel ink performed the same in my tests, that's my daily non-fountain ink). I only use these for writing, not drawing, so can't comment on that use; some seem to say that water bleeds them, but I can only suspect that it's if applied immediately after laying down the Seiboku. Once it dries, in my experiment there was no bleeding or ink loss whatsoever (but remember, I allowed the inks to dry for about a day before exposing them to those solvents). Some note that the ink dries and gums up the pen or is hard to clean. I've been using mine almost exclusively in Platinum pens (e.g Prefounte, Plasir; what can I say, I'm practical and go for nice, but inexpensive, pens!) and they have a proprietary cap that seals the nib tightly. All I can say is that I've had a converter filled for more than 2 months and the ink flows immediately and nicely with those pens/nibs! I haven't had any issues cleaning. I can't say the same when I've used other of my pens (for example Lamy Safari) - some without the nib seal have required a little work to restart if there's been a couple days since last use. All in all, one of my favorites! Best wishes!

I have been using this ink for years now, and won't use anything else, one reason is it's resistance to feathering on cheap copy type paper we use at work...for me, this works very well on cheap paper. Nice solid black color, flows well, looks great on the page...does everything well. I hope Sailor never discontinues this ink! Despite the "pigment" description, I have never had any issue with clogging feeds or nibs, and I've had pens sit for months with this ink, unused, and a little drop of water restores flow, like it never happened. It's worth mentioning that this ink has a distinct odor, it's not bad, but smells more like chemicals than the typical fountain pen ink...maybe that is its secret! Also makes it easy to figure out if it's Kiwaguro in the pen, just sniff the feed! Can't say enough good about this ink! A+++

I really like this ink. I use it mainly for my journal. It’s super smooth and the ink isn’t too watery. It doesn’t smudge easily either and dries relatively quickly. I will say though that it dries a little more grey than I’d like on paper, but I don’t mind it too much.