I'll admit that I prefer Noodler's Heart of Darkness over Noodler's Black because I think HoD dries a little quicker. Some people have complained about Noodler's Black being smudgy (the problem with the "bulletproof" inks is that they are cellulose reactive -- that means they chemically bond with the paper, but if you lay down a really wet line only the bottommost part of that wet line is actually going to touch, and therefore, bond, with the cellulose in the paper; it also can mean that it could bond with your cotton shirt...). I didn't particularly find that to be the case, but the ink didn't wow me, either. And since I'm not a great fan of black inks in general, it had better wow me at least a little bit.
While I haven't tried them all, here's my take on the various Noodler black inks:
Noodler's [bulletproof] Black -- "meh" IMO, but supposed to be good on bad paper
Noodler's HoD -- faster drying than the regular Black also pretty waterproof.
Noodler's Old Manhattan (exclusive to Fountain Pen Hospital in NYC) -- super black, but part of that is because there is more spread.
Noodler's X-Feather -- supposed to be better on really cheap paper but slower drying (I have a sample but haven't gotten around to trying it.
There are a lot of other ones that I don't have/haven't tried: Borealis Black is supposed to mimic Aurora Black; Bernanke Black is supposed to be super fast drying (for left-handed writers); Bad Black Moccasin is part of the Warden series (forgery resistant; Eel Black is supposed to be more lubricated (so good for piston filler pens); Polar Black is good for low temperatures (I guess if you're doing scientific notations at someplace like McMurdo Station...); Dark Matter is supposedly reverse engineered to be like vintage inks, with some water resistance.
I also know of at least two other "exclusives": Texas Black Bat (exclusive to Dromgooles's) and Raven Black (which may be exclusive to Wonder Pens in Toronto). I don't recall what is special about those beyond exclusivity; I passed on Raven Black when I was at Wonder Pens several years ago, on the grounds that it didn't sound ENOUGH different from what I already had.
And I seem to recall mention somewhere along the line of something called Brahmin Black, which was either a pen show exclusive or exclusive (at some point) to Bromfield Pen Shop in Boston.
Me? I like Heart of Darkness. I also have a bottle of Old Manhattan, just because the first time I was ever at FPH, I felt guilty about not finding the blue-black ink of my dreams so I figured "Hey, a lot of people call it Old Manhattan Blackety-Black...." But I never seem to use it (mostly because of the spreading).
Ruth Morrisson aka inkstainedruth
"It's very nice, but frankly, when I signed that list for a P-51, what I had in mind was a fountain pen."