When black beans are cooked their antioxidant content is approximately 75% less than what the raw beans contain. Regardless, they still provide much more antioxidants than many other staple foods you would eat similar amounts of (e.g. grains like rice, wheat flour).
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