I get what you're saying about people voting against their own interests, but I also think it's about many black voters feeling there aren't any really good options and dividing their votes accordingly.
It's also one of the reasons I say that positive campaigning can be effective, which we see playing out as Bloomberg hits the media with targeted ads, while Sanders, Buttigieg and others do little to nothing (comparatively speaking) with regards to trying to attract African-American voters.
Even Biden is getting a firsthand lesson on how he can't coast to the finish line on goodwill alone.