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.
The Black and Tan Coonhound is a large, determined hunting hound. The head is well-proportioned to the body. The length of the Black and Tan's body is equal to or slightly longer than the height of the dog at the withers. The topline is level. The relatively long head has an oval outline. The muzzle is long.