In addition to electability, there do appear to be other reasons African-American voters are going for Biden. They have a preference for politicians who offer incremental change, which may be one reason they didn't go for Sanders.
http://nymag.com/intelligencer/2016/...ck-voters.html
In addition, they do like Biden. They see him as an important ally, who helped the first African-American President get elected and succeed.
https://www.theatlantic.com/ideas/ar...-biden/589402/
Columnist Keli Goff summed it up on an appearance on Left, Right and Center.
“To succeed in primarily nonblack spaces––you only do it when you have really good allies,” she declared. “And I think that one of the things Biden gets credit for, rightly or wrongly, is this idea of being the blue-collar white guy who helped give Obama legitimacy with some of the blue-collar, white, male voters who voted for Obama-Biden and then crossed over for Donald Trump. I think African Americans give credit for that. Especially when the Obama campaign was really struggling, not always fairly, with some of these race-baiting attacks, sending Biden out on the campaign trail to fight some of those fights made a difference.”
This might not last, although if the Top 4 in the Iowa polls are in the top three when the caucuses come around, he might be the main beneficiary of their support, since he has more appeal to this group than Sanders, Warren or Buttigieg.