Black Mustard Seeds, Brassica nigra, are also called black mustard, brown mustard, or true mustard. These tiny(approx 1-2 mm in diameter) but strongly flavored reddish brown seeds are used in quite a few Indian cuisines although they are most pungent of all mustard varieties. There are more than 40 varieties of mustard plants, but culinary mustard seeds come from only three of these varieties.