I'd heard about discrimination against black cats (and about people adopting them around Halloween so they could torture them) but never anything about black dogs. So ridiculous. The only reason I can see not to have a black dog is if you have white carpet and furniture.
Although---my black cat sheds on my white clothes and my mostly white calico sheds on dark clothes. Such smart kitties to be able to coordinate how they shed!
All four of our pets are rescue critters. The black cat was likely dumped in our neighborhood. We passed out flyers, put ads in the newspaper and such but after a week, we realized that she was going to be our cat. Took her to the vet and he sort of yelled at me, asking when the cat had gone into heat. Turns out that when he opened her to be spayed, she was about two weeks pregnant with eight kittens. She's a very noisy cat and I'm sure her heats were loud and annoying so that's probably why she got dumped.
The dogs are probably sisters---they were abandoned by a couple who got a divorce and lived around the small town my DH lived in at the time. They hung around the church and the parsonage and finally convinced him that they were HIS dogs. However, before he adopted them, they got pregnant---probably by the same dog---and had little black puppies. Sounds like an episode of the Jerry Springer show! (Ironically, my husband was married to his ex-wife at the time---they later divorced and the divorce was quite traumatic for the dogs.)
Our other cat is a dilute calico, though we often refer to her as a deluded calico. The folks at the shelter were SOOOO happy that she was adopted out---at first I thought it was because she needed a home but then found out that she had been quite the terror in her foster home. She also gave birth before she was adopted and the shelter people named her little black male kitten "Lucifer", probably expecting him to have the same boisterous attitude as his mother. Silly cat---she thinks she's a dog and the dogs sometimes act like cats. We have weird (but much loved!) critters!
BTW, when I lived on a farm, we had no cats until a neighbor gave us a pregnant black and white female. She gave birth to two black male kittens, one black female and one black and white female. Before we moved to that farm, none of the neighbors had black cats but after our two toms were old enough to roam around, quite a few farm families had black kittens! The one cat was stereotypical tomcat mean but the other was a sweet, goofy, gentle wuss. Having my black cat now reminds me so much of the farm cats, especially the big, goofy Fred.
Getting back to the topic at hand, maybe a reason people don't adopt black dogs is because so many black dogs are big: labs, Newfounlands, Chows, pit bulls. I'm not thinking that many all-black dogs are small---daschund and maybe Corgis, but any others? And there aren't that many all-black (or all-white dogs) anyway, are there? One of my puppy girls is cream and tan and the other is shades of brown, mixed with black. (I told her she has a nose like a Siamese and hope she understood that it was a great compliment!)
Anyhoo, interesting thread!