I could see how only being critical of Obama would deter black voters, but that’s not what’s happening here. The people criticizing Obama for giving speeches also criticized Hillary Clinton for the exact same thing.
They didn’t criticize Sanders, but that’s because he has a boring retirement investment at 75, and it’s value is in the neighborhood of a few hours of speeches. Seriously comparing the that to paid speeches makes you a hack.
Furthermore, claiming that the people with these views as “far-left” is a joke. The far left is small, and has very little power. The mainstream of the Democratic party would be center-right or just right in other countries, but the country went crazy after Reagan, and the establishment Overton window kept shifting rightward. What you call the “far left” is often the policy of pre-Reagan Republicans.