Next to Sailor's excellent blue-black, and Noodler's deep, deep dark and, nearly, permanent, blue-black, Pilot's Tsuki-yo Iroshizuku (Colors of Nature) ink is certainly one of the best blue-black inks that are currently available.
Tsuki-yo, literally Moon-like ("tsuki" means moon), is "Moonlit night," a more appropriate translation (a beautiful Japanese idiom). The ink has a lovely intensity and richness that makes many other blue-black inks look watery and weak.
Currently, Pilot manufactures a pretty, well-made fountain pen to match this color. It is a Custom Heritage 91 model to match Iroshizuku Tsuki-yo. The pen retails for 10,500 Yen in Japan which is about U.S. $115.00. I have one and it writes very well with its 14K rhodium-plated nib. I recommend that JetPens market this model and the other Iroshizuku Custom Heritage 91 pens in Yama-budo, Yama-guri and Kiri-same (models available as of Feb. 2010). More colors will be coming out with barrels and caps made of plastic resin that match the various ink colors, and clips & trim are rhodiumplated steel. They are wonderful writing instruments to go along with some of the most beautiful inks ever produced.

The Iroshizuku inks are my absolutely favorite inks on the planet. This shade is so well reviewed but I’m with the lone reviewer who finds this one washed out and overrated. Tsuki-yo is soooo gorgeous in the bottle (and even on my fingers when I’m careless with a refill), but the pay off on paper is simply not for me. No matter which pen/paper combo I use, the effect is a denimy gray that makes me rather sleepy. Not easy on the eyes and writing an entire letter with this is laborious. Perhaps it’s just my particular bottle but I’m unwilling to repurchase to find out. My daily ink is Ku-jaku and though I find that Tsuki-yo shades better, there is 0 sheen here and the gray undertones exacerabate the matteness.

This was my first luxury ink buy and I've already gone back to buy another color from the line!

I've used Noodler's inks in my art for a few years and while they've done well, I'm going back to college and wanted something higher quality. This is the one I settled on and I couldn't be happier! I barely know where to start. Firstly, it's an extremely wet ink, at least for me. As I use a fine nib, rather cheap pen, this works out wonderfully and makes for a smooth flow with just a bit of slip as you write. I've had no issues with clogging or skipping, yet it also doesn't pool too badly if you rest your pen for a moment. It also hasn't feathered or bled through the paper I've been using, and the ghosting is minimal even on thinner paper.

As for the color, it is beautiful. It's blue-black, with subtle green undertones. It shades a fair amount, even on fine writing. You can get decent range of color and saturation depending on how much pressure you use. That doesn't detract from how professional it is - you can comfortably use it for both personal and business purposes.

It's honestly a wonderful ink, and if you're willing to spend this price on an ink, I would definitely recommend this one.

My favorite ink. There's just something about it that is amazing. I find myself putting it in all my new pens now! Never ever used to write in blue ink until I started using fountain pens. There is some feathering on lower quality papers; but that is most likely the papers fault. Great ink and worth the higher price- though I'd still like to see it a few dollars cheaper.

Wow. Amazing color, great bottle design, and pretty much fantastic everything else. More of a blue-black ink, the tinge of green in it makes for a unique color and change among more traditional blue-blacks. This is a great color to use in the workplace as well as just for fun. It's super smooth and the color is vivid and intense like the rest of the Iroshizuku line, and definitely shouldn't be missed.