For several months, radio host Tom Joyner has pleaded with his 8 million listeners to get in line behind the first black president. “Stick together, black people,” says Joyner, whose R&B morning show reaches one in four African American adults. The Rev. Al Sharpton, an ally of President Obama who has a daily radio show and hosts a nightly cable television program, recently told the president’s black critics, “I’m not telling you to shut up.