Black is just that, black. "Blue" Dobermans are actually a shade of gray color. Some of them are very dark, and can almsot be mistaken for black, and others are silvery, and some are all the shades in between. If you're in doubt, looke at the nose. The nose of a black Doberman will be black, and the nose of a blue Doberman will be grayish blue.
I get your point...Duchess is black...but usually in pictures she will have a Blue undertone...but its not at all part of her actual color...its just like hair dye...how there is a "BLUE BLACK" or a more natural looking black for human hair lol. Like when I draw duchess from a photo...I end up using alot of blue color in it for highlights...but she is for sure black dobe