Ok, we're not talking Stuart Semple black, but it was rumored long ago among painters that Holbein's peach black was the blackest black for oil paintings. Later we found Bruce Mac Evoy echo this in watercolor when he wrote that it has the darkest masstone.
If you're looking for blackest blacks in oils, make sure to do a lot of different tests as it is surprising how much black pigments vary in lightness. Matte vs. Gloss and particle size matter, and it's also helpful to look at them both varnished and unvarnished (once dried).
PBk1, also called Aniline Black, is sometimes used as an ingredient in blends and may be found in some oils along with PBk6, though whether that helps increase the darkness that much is debated. The addition of PBk1 to PBk6, which is carbon black, is interesting. Some sources suggest Analine black may not be super lightfast, though Carbon black is. Handprint suggests that perhaps one should not use this pigment in tints. Monona Rossol mentions in her work that some of the anilines are carcinogenic (see link below).
On the topic of blackest blacks, we recently heard that while research is still emerging, those cool nanotube black pigments like vantablack may actually have some serious health hazards associated similar to asbestos, so if you find yourself looking for blackest blacks, please consult Monona Rossol's work on artist safety.
