These videos would provide some insight:
https://youtu.be/Gy_FqMuehjQhttps://youtu.be/sJkSpJF5l3cLong story short---- she was the wife of a popular Democratic president (who was affectionately called "the first black president" before Obama) running against an unknown from 99% white Vermont, served faithfully under Obama, she had a "name brand" to run on. Black voters are loyal, they only broke away from her in 2008 when Obama's campaign began to look robust and legitimate.
ETA: I spoke with some elderly black voters when I did phone banking to notify them of the primary election date that didn't even know she had an opponent. LOL.