This is clearly pandering as already has been said, so I don't need to elaborate. One doesn't win the black vote without having previous experience doing so. Vermont is only 1% Black, which means Sanders has probably encountered very few black voters. When you are an unknown entity to a bloc of voters and only begin to reach out to them when you need their voters, it tends to be a very hard sell.
No. This is categorically not the reason. Unless you can prove it to me by showing me a way in which Hillary Clinton, who does have experience campaigning to Black people, is better at reaching Black voters than Sanders. Don't just say "she does better" I know that. I'm asking you a concrete example of HOW she's better than Bernie at talking to Black people. Unless you can give me an example of a specific way, it must be assumed that the reason for her success is name recognition/being married to Bill and nothing else.
It's all about name recognition, all about that. It's not about being better at talking to black people, it's about reaching voters without looking as if you are pandering or being condescending. Hillary Clinton is not a new name to many voters, including black voters and they know where she stands on most issues. In that sense, she doesn't have to pander.