"Black Dog Syndrome" or "Black Dog Bias" is a phenomenon related to the difficulties black dogs face when waiting to be adopted.
In comparison to dogs in other colors, black ones usually have to wait far longer to find a home.
The stereotype people have on black dogs describes them as more scary, violence, and aggressive, especially if they are also big and tall.
The reason for this discrimination is not clear, but the assumptions are that people have long-held negative superstitions.
Most people don't want to even consider adopting them just because of the dark coat, no matter how sweet-natured the pet is. For some reasons, most people just think the lighter colors are much more friendly, due to a kind of "beauty myth" for pets.
Some believe that this is just a myth and other factors are more relevant when choosing a pet, like age, size, and breed.
But if you ask the people working in pet shelters and rescue centers, probably most of them will tell you about the sad situation of the black dogs, some of them are never adopted, and often among the first to be euthanized.