Elizabeth Warren, dogged by weeks of doubts over whether she used her claims of Native American heritage to advance her academic career, now faces skepticism from Boston’s black ministers. The leaders are demanding a meeting with all the candidates for US Senate and saying Warren can’t take the black community’s vote for granted, despite her endorsement by the state’s black governor last week. But some Democratic strategists have called Republicans’ relentless focus on her heritage race-baiting, aimed at white men frustrated by the economy.