You've gotten some good answers.
In addition, it is not easy to get a good photo of a black dog, even in ideal conditions (I know -- I have one, and my favorite pics of her are against snow!).
And, even in person, it is often difficult to see their facial expressions, unless they are grinning outright -- dark eyes in a black face, hard-to-see muscle movement, nose blending in with the rest of the face, head blending in against the body. That, subliminally or openly, makes some people see less that is interesting or appealing in their faces.
Greyhound adoption folks often work to promote the black dogs.