WASHINGTON — When a handful of liberal advocacy organizations convened a series of focus groups with young black voters last month, the assessments of Donald J. Trump were predictably unsparing. But when the participants were asked about Hillary Clinton, their appraisals were just as blunt and nearly as biting. “What am I supposed to do if I don’t like him and I don’t trust her?” a millennial black woman in Ohio asked. “Choose between being stabbed and being shot? No way!” “She was part of the whole problem that started sending blacks to jail,” a young black man, also from...