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They're not all the same, their appearances are all very similar however only N.Sativa is known for medicinal and culinary use https://en.wikipedia.org/wiki/Nigella

Nigella
Nigella is a genus of 18 species of annual plants in the family Ranunculaceae, native to southern Europe, north Africa, south and southwest Asia. Common names applied to members of this genus are nigella, devil-in-a-bush or love-in-a-mist.
The species grow to 20–90 cm tall, with finely divided leaves; the leaf segments are narrowly linear to threadlike. The flowers are white, yellow, pink, pale blue or pale purple, with five to 10 petals.