Louise, I have been a volunteer at our town's animal shelter and involved and interested in animal rescue and welfare.
One of the reasons black cats and dogs are usually harder to adopt out is because their facial expressions are harder to see/read at first glance and also in photographs that are posted on pet adoption web sites. Much like how an all black outfit will lose detail in a photo and always looks better with strong texture.
There are black dog rescue organizations that will take the time to put a colored scarf on a black dog for their adoption pictures and make sure that they are photographed in good lighting with a nice background to give these animals an extra boost. The lighter fur dogs and cats, especially the bi and tri colored ones have that more instant appeal factor to people IRL and especially in photos and in dark kennel cages, because the color contrasts of fur and eyes reads as 'personality.' Ever notice how dogs that have the lighter colored 'eyebrows' against a darker coat seem to have a more expressive face? It's that ever important first impression that helps many animals get new homes.