Black mustard seeds are tiny little strong flavored seeds used most commonly in Indian cooking. They are the most pungent of all mustard seeds. The tiny seeds are dark brown with a slightly reddish hue to some seeds.
You can find black mustard seed in some very well-stocked grocery stores as well as any Indian market. You can also purchase Black Mustard Seed
If you don't have black mustard seed you can substitute equal quantities of:
Other varieties such as yellow mustard seed (larger and less pungent) OR> Brown mustard seed OR> Alternately you can substitute 3/4 to 1 teaspoon mustard powder per teaspoon mustard seeds OR> Use 1 tablespoon prepared mustard and reduce the liquid in your recipe by 1 teaspoon.