The tiny seeds of black mustard [B. nigra] are very strong, with a distinctive flavor. Black mustard seeds are commonly used in Indian cuisine, especially in curry recipes, where it is known as ray. Whole mustard seeds are frequently added to marinades and curries.
Black mustard is native to the southern Mediterranean region of Europe, where it has been cultivated for thousands of years. Despite their similar common names, black mustard and white mustard (genus Sinapis) are not closely related. Black mustard belongs to the same genus as cabbage. Ancient Greeks used the black mustard for treating lung congestion. When traditional medicine calls for "mustard plaster," black mustard is what they're referring to. In modern day Europe, ground black mustard seeds and honey are used as cough suppressant.
In Indian and Southeast Asian cooking, black mustard seeds are typically heated in hot oil or ghee [clarified butter], releasing a nutty flavor when they "pop." Frying changes the character of the seeds so that they taste nutty and mild. Frequently, whole black mustard seeds are added to marinades and curry recipes. Try black mustard with coconut shrimp, in red lentil dal, pork vindaloo, and lemon rice.