Black Dog Syndrome is defined as "the propensity of dark-coated animals to be passed over for adoption [at animal shelters] in favor of their lighter counterparts."
Reasons why the syndrome may exist: the age-old association of light with good and dark with evil; "Black dogs often don't photograph well. Facial features disappear, and animals can appear less expressive"; "black dogs sometimes fade away into the kennel shadows".
And apparently black cats face the same problem as black dogs.
However, there's debate over whether this syndrome is real. Many shelter and rescue leaders insist it's real, but skeptics note that the statistics don't seem to back up this perception. The general manager of the LA Animal Services Department notes: "In the last 12 months... 27% of the 30,046 dogs taken in by his department were predominantly or all black. Of those that were adopted, 28% were predominantly or all black." Link: LA Times.