PBk1

Ok, we're not talking Stuart Semple blackest-black, but it was rumored long ago among painters that Holbein's Peach Black was the blackest black for oil paintings. While it's not pure PBk1, it's one of the few black oil paints that includes PBk1 in the blend. Later we found Bruce Mac Evoy echoed something similar in watercolor when he wrote that it has the darkest masstone. However PBk1 may not have the best lightfastness.

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 once dried and the overall particle size of the pigment may make a difference. 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.

In terms of toxicity, we're not experts but Monona Rossol mentions in her work (available here),that some of the anilines are carcinogenic. 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.