This is a problem that I truly did not know existed , called Black Dog Syndrome:

“Did you know that Black Dogs are generally the last to be adopted and are euthanised 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.”

For more information and how you can help promote the adoption of black dogs, refer to the following links:

http://www.thebark.com/content/black-dogs-face-hard-choice-shelter

http://www.startseeingblackdogs.com/