Forest matches each pixel with the most similar color in the Geometry List, but black color has not any special meaning for the diversity map. So, in this case it works just because blue is the most similar color to black.
Anyway, i suggest to not use pure black (0,0,0) as color ID in the Geometry List, because you would get unexpected results editing the listbox. Instead, (1,1,1) or any other color will work fine.