Certain specimens of Arisaema triphyllum have almost black flowers, though there is massive variation across their range
(https://plants.ces.ncsu.edu/plants/arisaema-triphyllum/)
Also there’s Arum palaestinum, which is in the same family as A. triphyllum, so I wonder if there’s a connection?
And another I found, Fritillaria persica has almost-black flowers too.
https://en.wikipedia.org/wiki/Fritillaria_persica#/media/File:Fritillaria_persica_inflorescence.jpg
And i know there’s a few species of wild hellebore that are black, at least, in part, which is probably why its so easy to find fully black cultivated hellebores
EDIT: Thought of another one - certain Trilliums are so dark red that they almost look black in the right lighting. Given that they’re understory spring ephemerals, that right lighting isn’t too uncommon
https://www.wildflower.org/gallery/result.php?id_image=75195