The difference in Biden vs. Sanders is a top-down approach to politics (endorsements, media coverage, partnering with moneyed interests) vs. a bottoms up approach to politics (door knocking, phonebanking, rallies, etc.). The Biden approach happens to work better right now, which is why he did so well in the past few days.
And while obviously there are a lot of white people that go over the top with their hot takes concerning black voters, I don't think we have to swing to the opposite direction and pretend like Joe Biden has this completely organic, long-time support of black folks. He ran for president twice before, and didn't get support from anyone, let alone black voters, and then he got picked by Obama to be on the ticket so that Obama wouldn't be seen as too much of a newbie/radical, and he's now benefiting from that, which allows him to avoid having to actually do any campaigning or discussion of his actual record on black issues.