The seed of a small herb, (Nigella sativa). The plant grows to a height of 45 cm (18 in), has wispy green foliage, similar to dill and attractive white to pale blue flowers which ripen into the seed capsules. It is a close relative of the pretty ornamental flower love-in-a-mist (Nigella damascena).
The seeds are tiny, only 2 mm (1/8 in) long and are a deep jet black in colour, leading to its widespread mis-identification as black cumin seeds and black onion seeds, both of which belong to entirely different families. It is widely cultivated in India, the Middle East and North Africa.
Nigella seeds have a faintly bitter taste, due to the presence of nigellin, but also have delicious pungency and vivid aroma when cooked. It is Indian cuisine to which the seed is mostly linked, as an essential ingredient in the Bengali spice blend, panch phora, but also as a topping for naan, as well as scattered through rice pilaus. It marries very well to seafood dishes and is often cooked on top of the Turkish bread, pide.
On the medicinal front, there is no shortage of feasible to wild claims regarding the seeds curative properties, some of which include
Stomach complaints, especially diarrhea and dysentery
As a stimulant
Skin complaints, including psoriasis
Treatment for scorpion stings
Insect repellant for clothing
There is an Arabian proverb, 'in the black seed is the medicine for every disease except death', which seems like a bit of a wide call, but serves to demonstrate how this fascinating spice is widely regarded as close to a panacea.
For additional information, have a look at Kalonji.