I think I first heard of the "black dog" reference in relation to Winston Churchill. I knew immediately what they were talking about and I have taken to using the metaphor a lot in my own life.When Rowling used the concept of "The Grim" in Harry Potter I thought that that was a direct reference to the "black dog" in the sense of dread or depression.Swimming with the black dog can be therapeutic or dangerous, depending on the depth.
Blog of the Hand 
I think I first heard of the "black dog" reference in relation to Winston Churchill. I knew immediately what they were talking about and I have taken to using the metaphor a lot in my own life.When Rowling used the concept of "The Grim" in Harry Potter I thought that that was a direct reference to the "black dog" in the sense of dread or depression.Swimming with the black dog can be therapeutic or dangerous, depending on the depth.