Many black voters don't like it when you criticize Obama
Sanders' problems are three-fold: one, for all his high-flying rhetoric, he has little to show for his quarter century in national public office (Sanders first came to Washington as a House member in 1991). Secondly, and more importantly, Sanders is too interested in portraying himself as a drag-out "fighter" than a pragmatic doer. Third, his contempt for President Obama's painstakingly negotiated trade deals is a direct insult to minorities.
You cannot go after the oligarchs without going after, at least by implication, going after Obama's record. So I don't know what to suggest, except that Bernie's team will have to find a way to reach out to the black community. Giving interviews to black media outlets would be a good start.