Both Cumin and Black Cumin derive from the Apiaceae genus of plants, and both are edible. Taste-wise, however, they are quite different from one another. Cumin in general is a popular spice used in Latin-America, the Middle East, Southeast Asia and India for many traditional (and delicious!) regional dishes.
Common Cumin (Cuminum Cyminum)
Originating in the Mediterranean, cumin is one of many seeds used by ancient Romans as both a spice and a medicine. The Egyptians also had a use for cumin, although it was neither culinary nor medicinal; they used it mainly to preserve bodies before mummifying them! Cumin does have preservative qualities, which is one of the reasons it's often found in meats like sausage.
One of the earliest medical uses of cumin was in India, where they used cumin to enhance digestive health, a clue to why it's called cumin, which translates to "that which aids digestion". Even today, in East India, people still boil cumin seeds in water and use them as a medicine for edema, heart disease, stomach ailments, vomiting and fever. Cumin is said to soothe intestinal spasms and remove gas from the stomach and intestines, as well as stimulating the appetite.
But anecdotal evidence is backed up by scientific evidence about the digestive and other curative powers of cumin. Cuminaldehyde is the natural substance found in cumin seeds that gives them their distinctive nutty, lemony, peppery, slightly bitter taste. Cuminaldehyde contains extremely potent antioxidants which studies credit with preventing foodborne illnesses and improving symptoms in patients with IBS (Irritable Bowel Syndrome), among other things. Black cumin seed (Nigella sativa) is also believed to reduce the risk of certain neurodegnerative diseases like Parkinson's.
Black Cumin (Bunium bulbocastanum or Nigella sativa)
There's a bit of confusion in play when speaking about black cumin, since two different plants -- Bunium bulbocastanum and Nigella sativa -- are both commonly referred to as black cumin. More confusing still, the seeds of both plants are prized for their medicinal qualities!
Bunium bulbocastanum is native to several regions in the world including northern Africa, southeastern Europe and southern Asia. Sometimes referred to as "soil chestnut" or "black zira", it grows to a height of approximately two feet, and bears white flowers that look a little like Queen Anne's Lace, similar in looks to Cuminum cynimum (common cumin). Every part of the plant is useful -- the leaves as a herb, the roots as a food reminiscent of chestnuts, and the seeds for their medicinal qualities.
To date, there isn't an extensive body of research on bunium bulbocastanum, but some studies suggest that it may be effective in several areas such as fighting certain bacterial infections which don't respond well to antibiotics, as well as possibly reducing the effects of diabetes complications resulting from oxidation and glycation.
More research is needed on the medical uses of bunium bulbocatanum, but there is more extensive research available on "the other" black cumin -- Nigella sativa. More commonly referred to as "black seed", this plant is native to the same regions of the world as Bunium bulbocastanum, but grows taller -- to three feet -- and looks completely different with small pale flowers and pods filled with seeds. Its coal-black seeds are matte or dull, and shaped like tiny Brazil nuts. Various studies show it promises to be helpful in reducing the size of cancerous tumors, enhancing memory and cognition in Alzheimer's patients, and reducing nasal allergy symptoms.
Both are Useful
Common cumin is both healthful, tasty, and medicinally useful. It's also nutritious, due to its abundance of vitamin C and vitamin A. Cumin also contains dietary fiber and has stimulating, anti-microbial and anti-fungal properties. Finally, cumin is a great source of iron, an important mineral.
Black cumin is more bitter and may be a bit strong and bitter for American tastes (It's used in specific dishes in northern and western Indian cuisine). But the medicinal qualities alone make it a useful herb, even if you just munch on its seeds or use them to make an herbal tea! Whether you choose common or black cumin, you'll be adding something really special to your herb garden!