One of the more unique nutrients found in mustard seeds is selenium. Just 2 teaspoons (4 grams) contains 8.32 mcg, which is 15% of your daily recommended intake. Another unique nutrient, omega 3 fatty acids, is also present. The same 2 teaspoons provides 0.15 grams of omega 3's, which is equal to 6% of your daily value. Lastly, the overall antioxidant content of mustard seed spice is over 6x that of blueberries, when compared on an equal weight basis.
ORAC Source
USDA Database for the Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 - Prepared by Nutrient Data Laboratory, Beltsville Human Nutrition Research Center (BHNRC), Agricultural Research Service (ARS), U.S. Department of Agriculture (USDA) - May 2010