I actually wrote an article about the black vote. Its on a medium account I don't use anymore though. Sanders was getting ~34% of the vote in his best states, like Missouri and Illinois. There were three major reasons Sanders didn't do better. One was time. He wasn't originally running as a real contender and he hadn't done a big tour of the South so people didn't know who he was.

Secondly for various reasons its much harder for a lefty, non-establishment candidate in general in the South and further Hillary used her political connections to suck up the black thought leader vote.

Finally because nearly all African Americans who vote vote Dem, there is a large amount of votes that are basically conservative and therefore support Hillary's more centrist economic message and some of her more centrist social messages. Basically Sanders couldn't expect to win those votes ever basically. Hence getting 34% in his best states is almost equivalent to getting like 52% of the white vote.

But the campaign itself made bad choices and we heard it at the door in Missouri and from campaign staffers.

Most of these problems don't exist for 2020 since Hillary is done, Sanders is well known now, he's done work on his black outreach skills and he'll have more time and money should he decide to run.