[You can click on any of the photos to enlarge for more detail]
Goulet Pens had a Samplepalooza sale a few weeks ago and I indulged with 20 ink samples, including these waterproof blacks. I figured 20 samples was a whole lot better than 20 more bottles.
I've been using Platinum Carbon Black for about a year for my art and thought that I would check out some of the other blacks to see if one might compare, perhaps for a few less dollars. Carbon Black in a wonderful ink, but it is a little on the pricey side, although the bottle is larger than some at 60ml/2 oz. Carbon Black is $19.00 per bottle at Goulet Pens and $25.00 at JetPens.
The paper used for the above testing is Neenah Classic Crest Solar White 24 lb. First I did a q-tip swab of each ink and then wrote the inks name next to it (I used a glass dip pen). The swab tests show that the blackest inks are the Noodler's Polar Black and Noodler's X-Feather.
Five of these eight inks are Noodler's all of which are "Eternal" inks and "Bulletproof". As you can see from the tests some of these are more eternal and bulletproof than others. Polar Black and Heart of Darkness are the best.
Sailor Kiwa-Guro went on the smoothest, for large applications this ink would give the most seamless appearance.
The Private Reserve Invincible Black is the old formulation. I emailed Brian Goulet to ask about the difference and his understanding is that the reformulation is related to an effort to reduce the cost of the production.
Edited to add: I failed to describe the torture test. I used a q-tip to scrub the written names and then I held the whole thing under running water for a few minutes.
Conclusion: Platinum Carbon Black is still my top choice, but when I need a new bottle of ink, I will probably get Noodler's Polar Black, the price $12.50 for 3 oz or $18.00 for 4.5 oz so their is considerable savings there.