"There's something about this that's so black, it's like how much more black could this be? And the answer is none. None more black." ---Nigel Tufnel, composer, describing Aurora black ink.
Last night, I was wondering, what if I took a look back at the writing that I've done over the four years or so since I've been an avid fountain pen user, and tried to select the best black ink I've used? The answer to any question like this is obviously subjective, because people value different things in a basic black ink. Some want darkness of color, where no shading or hints of other colors are present. Some want absolute permanence. Others value the ability to purchase the ink in massive quantities, such as a giant custom-ordered bottle of Noodler's or a liter bottle of Pelikan Brilliant Black. As for me, I'm not a heavy user of black ink, and only have two bottles. The first, Sailor's Kiwa-Guro Nano Black, is a pigmented ink that meets my occasional need for permanence. It's a pigmented ink, however, which is specifically designed for use in very fine Japanese nibs. I've found that if you use it in a broader nib, especially a wet nib, the pigment "stacks" or "pools" on the paper as the ink dries and it will smear or ghost onto the next page.** Therefore, for most day-to-day writing, if I'm in the mood for a basic black ink, I choose Aurora black because it's a "true black" and simply works well. Aurora black is a very dark ink, which dries quickly, flows well, and has worked in every pen I've ever used it in, even the most temperamental of vintage writers. Some people turn to Waterman Florida/Serenity blue to troubleshoot a pen; I turn to Aurora black. I highly recommend this ink as a daily user. The only caveat is that you may see some feathering and bleeding on very cheap copy paper, but that will be minimized in a fine nib. On moderately heavy paper, you should have no problems.
** Noodler's Bulletproof black and Heart of Darkness had the same smearing/ghosting issues for me. People have recommended diluting them with distilled water, but since permanence is not a serious concern for me, I've stuck with Aurora and saved myself the trouble. In situations where true permancence is required, I'll happily use ballpoints, a Sharpie pen, or a Sakura Pigma Micron.