The name of the black accent color is "cobalt." The cobalt theme was, I think, first seen in a video of the HTC 8s. Naturally everyone said black. I saw a video today of the lumia 820 with this same theme but the accent color was called "cobalt."
Lumia 820 shown with cobalt theme
Settings showing theme name
Accent colors shown in the SDK
Lumia 820 shown with cobalt theme
Settings showing theme name
Accent colors shown in the SDK