
It is a phenomenon that, not proven factual, is still a problem. The definition given by Start Seeing Black Dogs:“Did you know that Black Dogs are generally the last to be adopted and are euthanized in shelters in much larger numbers? People may consider them too “plain”, think they are menacing, or simply don't notice them because their dark color makes them disappear in their kennels. Those involved in animal welfare have dubbed these phenomena "Black Dog Syndrome."
http://www.startseeingblackdogs.com/