Nigella seeds are small black seeds with a crunchy texture that bring an oniony, sour taste to a dish according to Nigella Lawson, author of "Feast Food To Celebrate Life." They're also known as black cumin seeds. Nigella seeds are about the size of sesame seeds. The seeds appear often in East Indian Cooking. Their flavor is enhanced by heating or baking. Look for them in specialty and ethnic markets. If your recipe calls for them and you've run out there are a few substitutes.
Celery Seeds
Substitute equal amounts of celery seeds as the recipe calls for nigella seeds. Add in about half as much onion powder and a squeeze of lemon juice. This mixture has the crunch from the celery seeds along with the sour onion taste.
Black Sesame Seeds
Sesame seeds do not taste like nigella seeds but, if the reason you're using the nigella seeds is because of their color, then substituting black sesame seeds will work. Use equal amounts.
Cumin Powder
Cumin is the pulverized form of coriander seeds. When planted, coriander seeds grow into a plant also known as cilantro. It's three spices in one, with three different tastes. Cumin has a more intense flavor than coriander. Coriander, when toasted, has an intriguing, nutty flavor balanced by a bite of acid. Cilantro has a bright-green, herb flavor favored in Southwestern dishes. The cumin powder is closest to the nigella seeds for flavor.
Onion
Grate an onion, strain and use the juice in place of nigella seeds. If short on time, place the onion in a blender on the liquefy setting and use the pulp.