I skimmed through a bunch of this thread after not looking in for a while.
Perhaps it's not so apparent, but Bernie and his campaign have been improving on the front regarding race issues since Netroots where he received a wake up call. I think the outreach and visibility takes a bit of time to spread even in this day and age.
However, it's not an excuse for the condescending attitudes from Bernie supporters about how Black Lives Matters or black people should act to effect change. Bringing up how Bernie was a civil rights activist in the 1960s and how he marched with MLK is good for information, but bad as an argument to shut black people up. It comes off with the impression that Bernie supporters are telling black people Bernie did his time already, let him focus on issues that speak to the rest of the voting population (essentially implying that race issues take a backseat), or worse yet, suggesting that because Bernie was a civil rights activist 50 years ago, he knows more about race issues than black people themselves.
They also do themselves no favors when their 'defense' of Bernie on race issues consists of attacking BLM, attacking other candidates, and claiming conspiracies and sabotage. If Bernie has merit on race issues, find his voting record, his list of actions to address race as mayor, etc. (Voting record will require some digging though, it's pretty long.) Attack on the basis of the quality of your candidate, rather than on the lack of quality in others.
On the opposite side, I think saying "that's not enough" is unspecific and unhelpful. It's clear Bernie's campaign has been working on the issue. When presented with what the Bernie campaign has been putting out (it's pretty detailed!), saying "that's not enough" gives the impression of not listening at all. That's an unsatisfactory answer that leads the people who are presenting a case to think you're not someone who is open to conversation at all.
I think ultimately what hurts Bernie the most is that both at Netroots and this rally, he opted to just back out as opposed to engaging. Defenders will claim he has no reason to engage with the unreasonable. But if you're going to back out when heckling and disruption happens, it's unpresidential. Obama, Hillary, O'Malley, Biden, etc. have experienced that kind of thing, and they didn't just back out. Even Romney and McCain. When you campaign to be president, even if you are white and male, you need a thick skin.
Also, the panic over the left eating itself is kind of funny. Do you see Trump on the other side of the fence? I wouldn't panic about the differences in opinion on the left until we have a Trump in here. I think these discussions are substantial and helpful in a way, because we probably should be talking about this. Rather than about what Donald Trump said about Megyn Kelly's nose.