Onyx Black is a black with a purple undertone, and black sheen.
In large swabs, the purple undertone really comes through. In this swab it almost looks like Halloween in a bottle with the purple undertone and black sheen.
In ink drops, the black sheen really dominates over the rest of the ink.
Feathering: Onyx Black only had a little bit of feathering on Baron Fig paper, but did great on the other papers.
Bleeding: Onyx Black only had bleeding on the large swabs.
Ghosting: Onyx Black had medium ghosting on all of the papers.
Shading: Onyx Black had low shading on all of the papers. The black sheen didn't show up on Rhodia, had low sheen on Baron Fig paper, medium on Tomoe River and Leuchtturm, and high on the large swabs on Tomoe River.
Top row: left to right, Robert Oster Graphite, Diamine Jet Black, and Diamine Onyx Black. Bottom row: Pilot Iroshizuku Take-sumi, Robert Oster Grun-Schwarz, and Pelikan Edelstein Onyx.
I think the closest ink to Onyx Black is Pilot Iroshizuku Take-sumi. It has sheen like Onyx Black does, but it doesn't have the same purple undertone.
Overall, I think Onyx Black has an average flow, and performs consistently among the different papers. It's a great black if you like a little bit of sheen to your writing, but I wouldn't call it a classic black because of the purple undertone. I still think it looks like Halloween in a bottle.
Disclaimer: I purchased this ink myself, and all photos and opinions are my own. This post does contain an affiliate link.