What's missing is the analysis of why this behavior is occurring.
It's the desired outcome of the Clinton campaign's cynical and underhanded propaganda: "Bernie Sanders has a problem with black people." It's clearly false, they know that it's clearly false, yet they push the message anyway because it results in exactly what the OP is lamenting: a pointless side-argument over endorsements and who is or isn't a better choice for black voters.
Clinton supporters know the argument is manufactured and meaningless but it keeps them from having to defend Hillary's bad policies, so they keep beating the drum. Which naturally invites a response. And who can really blame Sanders supporters for responding? A nearly universal complaint about Presidential Candidate John Kerry was that he refused to stand up and fight back against the Swift Boat Veterans for Truth when they spread lies about his military service. It's odd that we would criticize Sanders supporters for standing up to the Clinton campaign's lies.