I can't believe that I've had this ink for over four months and still have not posted a review. It's no secret that I love Sailor Inks, and the special line of inks that Sailor manufactured for the Japanese retailer Bung Box (in Hamamatsu) has developed a cult following. At last week's D.C. Pen Show, Bung Box attended with a suitcase full of ink in tow, and apparently sold all of it.
Frankly, BB Sapphire is the best blue ink I have used. The color pops, it doesn't fade, and the ink has all the great properties of Sailor ink. (When I refer to an ink having "great properties," or an ink that "behaves well," I typically mean that the ink dries quickly and doesn't feather or bleed on the cheap paper that I have to use at work.)
I have three bottles of Bung Box ink: 4B ("Bung Box Blue Black"), Sapphire, and Norwegian Wood (emerald green), the latter of which I picked up last week. All of my bottles are the special Bung Box tall version, as opposed to the shorter Sailor bottle with an internal inkwell. I've heard that Sailor is discontinuing the special tall bottles--at the D.C. Show, several of the inks that Bung Box brought with them were packaged in the regular Sailor bottles. There's nothing wrong with regular Sailor ink, but it can be difficult to fill a large piston-fill pen such as a Montblanc or a Pelikan using an ordinary Sailor ink bottle. Cartridge/Converter pens pose less of a problem, because you can fill the converter directly without even dipping the nib.
Bung Box ink is expensive. In the U.S., you can purchase from Vanness Pen Shop for $35.65 per bottle. Currently, their stock is limited. You can also try to track down contact information for the shop in Japan, and there are several eBay-based purchasing services who can obtain Bung Box Ink for you, but there will not be significant cost savings. The price per-bottle will remain around $35 (possibly $30 if you buy multiple bottles and can save on shipping).