Many of us are under the impression that kalonji and onion seeds are one and the same thing possibly because kalonji is also called black onion seeds. ... Kalonji seeds are obtained from the Nigella sativa plant while onion seeds are obtained from the Allium cepa plant. If you want onion flowers and seeds, leave a few plants in the ground until the following summer. Onion flowers are edible and have a sweet, strong onion taste. For seed harvest, allow the flower head to dry on the plant. ... Small black seeds from inside each tiny flower. onion seed (plural onion seeds) Used other than with a figurative or idiomatic meaning: A seed produced by an onion plant. The strong-flavored seed of black caraway or nigella, Nigella sativa, which is deep black with flat sides, like those of the onion. It's also believed that onions may boost circulation. Applying onion juice to the hair and scalp could increase blood supply to hair follicles, which in turn improves hair growth. There have been studies on how onion juice prevents hair loss, but not on its other benefits