Why would Black be reluctant? They seem particularly motivated up until now, and heavily favor Biden.
Yes, Black supporters support Joe Biden by a large margin and they don't support Bernie.
The black support for Hillary was low as well.
Voter apathy and Clinton's failure to connect with the black community -- particularly African American millennials disillusioned with establishment Democratic politics -- were also part of the problem.
Americans now understand the existential threat to our country and that is why "Vote Blue, No Matter Who" materialized.
Few people know that Sanders in 2016 actually edged out Hillary among white voters in the Democratic Primary. But with Hillary receiving close to 80% of the black vote, she won going away.
In the general election Blacks voted for Hillary by an 89-8 margin over Trump.https://www.cnn.com/election/2016/results/exit-polls
Not as good as they voted for Obama in 2012 93-6 over Romney.https://www.cnn.com/election/2012/results/race/president/
But Hillary wasn't black.
Further history of black voting.
2008 95-4 Obama over McCain
2004 88-11 Kerry over Bush
2000 90-8 Gore over Bush
1996 84-12 Clinton over Dole
1992 83-10 Clinton over Bush
1988 86-12 Dukakis over Bush
1984 90-9 Mondale over Reagan
1980 85-11 Carter over Reagan
1976 83-16 Carter over Ford
1972 82-18 McGovern over Nixonhttps://www.nytimes.com/elections/2008/results/president/national-exit-polls.html
As you can see the 89% support for Hillary is probably two or three points above the historical average of black support for the Democratic nominee. Trump receiving 8% of the black vote was lower than other Republican nominees except McCain, Romney who were up against Obama and Bush in 2000 which received the same 8%.