Black Hawk County is a county in the northeastern part of U.S. state of Iowa. As of the 2010 census, the population was 131,090, making it Iowa's fourth-most populous county. The county seat is Waterloo.Black Hawk County is part of the Waterloo-Cedar Falls, IA Metropolitan Statistical Area.HistoryBlack Hawk County was formed on February 17, 1843 from sections of Buchanan County. It was named after Black Hawk, a Sauk leader during the 1832 Black Hawk War.