Well, I still see no mention of "black seed" [1, 2] but I agree that the English Wikipedia does indeed mis-identify Kalonji as also being classifiable as "black cumin". Completely incorrectly, of course, but it does not directly support your hypothesis that Kalonji is /sold/ as "black cumin", although it is frequently (and equally wrongly) sold as "onion seed".
** Phil.
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[1] Black cumin
From Wikipedia, the free encyclopedia
Bunium bulbocastanum
Nigella sativa
Black cumin can refer to the seeds of either of two quite different plants, both of which are used as spices:
Bunium bulbocastanum, similar to caraway in shape
Nigella sativa, also called kalonji or nigella, and more common in the West
[2]Bunium persicum is a plant species in the family Apiaceae formerly included in genus Carum. It is related to cumin (Cuminum cyminum).
The closely related species Bunium bulbocastanum, which is used as a spice and as a vegetable, and commonly called great pignut,[3] black zira,[3] or earthnut,[3] was formerly considered to be a synonym of B. persicum.[4]