Since buying and using the Sailor Carbon ink that I bought in cartridges, I haven't flushed out any fountain pens.
But yesterday after a writing spree, my two Pelikan m200s went dry.
Now I can compare the five black inks I own: Aurora, J. Herbin Perle Noire, Noodler's Heart of Darkness and the two Sailor inks: Nano Carbon and Jentle.
The pens I used, from top to bottom: Sailor Sapporo, Sailor m1911, Pelikan m200 (Blue), Pelikan m200 (Grey) and Charcoal Black Lamy Safari
I tested all five inks on Staple's Bagasse paper with the results below:
With the naked eye it appears the Sailor Carbon Black and the Noodler's Heart of Darkness are the darkest. When I write without thinking of comparison I found the Sailor Nano Carbon just a tad darker than the Sailor Jentle.
The Aurora and J. Herbin blacks seem similar on this paper and just a little less saturated than the two Sailor inks.
The fountain pens and nibs I use definitely influence the way the inks respond.
It is very difficult to capture the written page so I gave up photographing the other samples as they do not significantly differ from the Staple's Bagasse.
Overall I don't find one of these five inks better than the others, all dried quickly, all are very responsive in four different fountain pens and on three types of paper (Canson cachet drawing paper, quality Clairefontaine Triomphe 90g tablet paper and the Staple's Bagasse). None of the inks seemed to feather or change the nib line.
Although I think the Sailor Nano Carbon is the darkest of the dark, I don't believe I will buy it over more accessible inks, notably the J. Herbin and Aurora. However, I am glad I purchased the Sailor Nano Carbon and especially pleased how much I appreciate a cartridge in the Sailor pens. The cartridges seem to have more ink than the converters and are easier to use.
I'd say you couldn't go wrong with any of these five inks and if price and availability are concerns the Aurora, J. Herbin and Noodler's are more than satisfactory. If you like variety, the Sailor Nano Carbon is a good choice.