What made you want to look up xenial? Please tell us where you read or heard it (including the quote, if possible).
Black dog is an oft-used phrase to mean depression. Here's a link that attributes the phrase to Winston Churchill, but I suspect he didn't invent it. I like the way they described it though: Black Dog was Churchill's name for his depression, and as is true with all metaphors, it speaks volumes.
Black Mouth Cur. The Black Mouth Cur is a hunting and cattle dog that has its origins in Southern United States. Contents. Description. The Black Mouth Cur is a well-muscled rugged herding, hunting, and all around utility (or work) dog whose coat comes in a number of colors and shades, generally all shades of fawn from nearly white to mahogany.