I am way too early in my ink exploration so I am hesitant to even say this ... but ... I think I have found my black ink.
While Nathan Tardiff may never say explicitly, the name alone should tell you that Noodler's Borealis Black was created as a response to Aurora Black, which is one of the darkest basic blacks on the market. I haven't tested Aurora Black myself, but I think any black ink will have a hard time matching Borealis Black in pure darkness.
This ink is not without its flaws, which means it is not for everyone. First of all, this is not a bulletproof black ink. According to Noodler's, this ink is moderately waterproof, but can be washed out and is not light resistant. For me, bulletproof is not high on my list of ink priorities.
Secondly, and more importantly to some, Borealis Black takes a long time to dry. In my sample, you can still see some smearing at the 30 second mark. That is forever in dry time. I can't imagine a lefty ever using this ink. For me, it is not an issue. Using the same Rhodia paper in the photo I haven't had any ink transfer to my hand or to another page.
What I do love about this ink is that it writes smoother in my finest tip pens than any other ink I have tried. The review was done with my TWSBI Mini 1.5 mm Stub nib and I was so enamored with the performance and feel I loaded it out in my Pilot Prera F nib pen. The results were fantastic. The flow of the ink from the nib to the page had a different feel - it was highly lubricated and flowed freely.
I have several more black inks to test and as of right now this is the ink that they will all be measured against.