"The combination of ‘blackness’ with the negative connotations of ‘dog’, noun and verb, seems an eminently apt description of depression: an ever-present companion, lurking in the shadows just out of sight, growling, vaguely menacing, always on the alert; sinister and unpredictable, capable of overwhelming you at any moment. Further, the ‘dark hound’ is an archetypal object of fear, with a long tradition in folklore and myth. Black dogs in dreams are interpreted negatively, often representing death; from all over the world come tales of nightmares caused by oppressive black dogs crushing the sleeper’s chest."
The first paragraph in the essay that won a competition — by the Black Dog Institute — on the theme of "the history of the term ‘black dog’ as a description for depression." Like many, I traced the metaphor to Winston Churchill, but he was a late entry in a long tradition.
The topic came up this morning at Meadhouse, which is brightened this morning — and many mornings — by the presence of Zeus, the neighbors' black Labrador Retriever. I've heard that there's prejudice against black dogs. They're least likely to be chosen from amongst the abandoned shelter dogs, most likely to get put down. Ah, yes: black dog syndrome. Some people jump to the conclusion that it's racist. But it seems more likely that people experience the black dog metaphorically. It's depression.
(Another really practical explanation is that black dogs don't photograph well, so it's harder for shelters to interest people in their black dogs. But since people love to photograph their dogs, maybe you should prefer a dog that photographs well.)