MSN had an article today about Black Dog Syndrome. Or, in layman terms, "I want a cute dog--one that you can see 'expressions' on" which directly translates to 'lighter coat (or face markings) means a better sell'
Of course, this is one of those things that unless you frequent an animal shelter, you're not going to know it exists. But it absolutely does. When I was doing the Specially Trained Adoptable Rovers class at the Boulder Valley Humane Society, there were a couple of times that our classes consisted entirely of all black dogs. And of course, it's never like you only had those dogs for one week. For about a month and a half every class consisted of Ritz (black lab/pointer), Chase (black lab/german shepard), Star (black lab), Willa (black lab/hound), Lily (black lab/pointer), Bear (black chow mix), and Josie (red foxhound/Australian Cattle dog) and then whatever other dogs were assigned to the class for the week (we usually had 9-12 dogs per class).
The article doesn't mention though that there are some other factors that tend to show up: The toughest dogs to adopt out are black lab mixes 1-3 years old. This is probably because lab mixes are notoriously energetic, and they reach full adult size at about a year. It's cute when you're puppy jumps on you when he's little, but when you've got a 60 pound dog running at you that you know is going to jump, it's just not nearly as cute.
Now then, with black cats, it doesn't tend to be as much of a problem--or rather it goes in cycles (one month the shelter will have tons of black cats, the next month, it's gray--and this just seesaws back and forth). I think it's because some people will go to the shelter specifically looking for a black cat, but hardly anyone goes in saying "I'm looking for a black dog"
So, if you're going in for a dog, my suggestion is to talk to the volunteers and the people that work there, they're the ones that know how the dogs act. And remember, when you have the meet with the dog, make sure you make the decision on how it acts, and not what it looks like.