But Julie Morris, the senior vice president of the American Society for the Prevention of Cruelty to Animals, says that while the Black Dog Syndrome is talked about a lot, there are no hard numbers to support the theory.
"There is no research data and no controlled research to support [the Black Dog Syndrome theory], which doesn't necessarily mean it's not true, but just that there is no data," said Morris.
"There are lots of compounding factors," said Morris. "Are there just more black dogs in shelters or is the real problem because most black dogs are also big dogs?"
According to Morris, the majority of people adopting dogs – especially those in urban areas – prefer smaller, easier-to-handle dogs which often times are not black.
"It's not really clear if shelters see this as a problem just because there are simply more black dogs out there," said Morris.
Even if the black dog syndrome isn't proven, Morris said that any adoption is a good adoption and there is no harm in these shelters drawing extra attention to dark dogs and cats.
Hancock is planning another special campaign to attract cat lovers to help deal with a recent influx of black cats at her North Carolina shelter.
"Human nature leads people toward things that are more vibrant and riveting in color," said Hancock. "It's very, very superficial... there is no inherent difference in these animals, they're just angels."