Hello Darkness is certainly very black. I have found Superstition to be more purple than black and dusky challenger is also very purple, just in case you were considering them. About Last Night is the closest to true black that I have ever seen, with the exception that it has a bright yellow beard.

If you go to the iris forum page at davesgarden.com, there's a whole thread on black iris with pictures.

BEFORE THE STORM, SAMBUCA, EBONY ANGEL are all dark iris.
There are some earlier ones such as INTERPOL and SABLE ROBE are justa s 'black', but are of a more tailored shape.
Laetitia

Matt McNames is the best here in MS.

Walter Moores

Badlands
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Before the Storm
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Black Phantom
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Black Suited
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Fade to Black
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Three of these show up as dark violet on my monitor and MM shows up with dark violet stds. and black falls.

The two 'sooty' blacks I have grown are/were BEFORE THE STORN and MIDNIGHT OIL - neither doing very well. BTS is extinct here.

Walter Moores

Before the Storm was taken with my old Kodak which for some reason had trouble with some colors at times. I know somewhere I have a truer picture. Somewhere I have a picture of Matt McNames.
Here is Old Black Magic and a poor picture of Midnight Oil. Colors are accurate. I use my camera to date bloom times. Both of these are Late Season here. This can sometimes be a problem because our weather usually warms signicantly in the late season leaving burnt edges on the bloom.
Midnight Oil
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Old Black Magic
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We have 3 "black" tall bearded iris. Many have a purple cast in our hands. But "Midnight Oil" looks the blackest so far (see photo of flower still opening). Schreiner's catalog suggests "Hello Darkness" is their best.