I think it has to do with several factors.
1. The incumbent opposition is black. I've heard the phrase "identity politics" thrown around since 2008, which apparently means if Hilliary Clinton runs it excites women voters as they get behind supporting the possible first female POTUS, if Mike Huckabee runs he guarantees a strong turn out among Evangelicals and its a good idea to have a someone on the ticket from a swing state because it'll help that party win that state. However when blacks are proud that in consideration of centuries of unfair treatment based on their ethnicity finally see people of all races elect a black man as POTUS and they want to get behind that, unlike the other examples I cited, this is the unethical 'identity politics' in action, 'using the race card' and is itself a form of racism, according to the cynics.
2. Since the early 1970s, blacks have overwhelmingly supported democrats. Blacks are to the democrats what Evangelicals are to republicans, their most loyal and often taken for granted constituency. If it had been Hillary Clinton, as with any other Presidential race black support for the democrat would be over 90%.
3. The LDS Church. Its perceived in the black community that the LDS church is racist. I haven't personally verified this but I saw a TV interview where it was stated that up until the late 1970s the official doctrinal position of the LDS was that black people were evil as a sole consequence of their ethnicity. In fact, to keep from mischaracterizing them, let me do a quick google search. I've gotten in trouble on boards before when citing other sources on controversial issues as if they were my own opinions I was trying to advance. ...
Mormon Prophet:Blacks Represent Satan On Earth - YouTube
I do not think Mitt Romney is racist but I don't think many blacks aren't as willing to separate the historic teaching of his church with from what they suspect are reflective of his personal views no matter how much he might deny it considering politics is a vocation built upon pandering, dishonesty and spin. I'm sure in 2008 many people had similar concerns with then Senator Obama's church.