Let's see what happens with other polls. I wish Bernie would sling a little mud here. While he was getting arrested in Civil Rights marches Hillary was campaigning for Goldwater. That's a fact and I don't see why it isn't fair game. At the very least, Bernie should be pointing out that issues most concerning to the black community when you ask them are his life's work.
I think the issue isn't that he isn't bringing it up, I think the issue is that he isn't bringing it up
enough. It should be a central point in his campaign, showing that not only do his views align with helping the poor and disenfranchised, but also that his actions back that up. One of his main points every time he speaks is how his ideas have not changed in 30 years (it's not really true, but it makes a good speech), and adding to that that he was marching alongside King can only help that.