Russia’s effort to discourage minorities from voting for Hillary Clinton — described in a federal indictment released Friday — revives the mystery of why turnout among black voters declined so substantially from 2012 to 2016. The indictment of 13 Russian trolls lays out a surprisingly sophisticated social media campaign to turn minorities, and black people in particular, against Clinton. Details of the campaign show that Russians understood both the significance of those voters in the election and the language of black social media. “Russians understood how important minority voters were to Hillary Clinton’s chances in this election. They were able...