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.
What if my dogâs urine is brown to black. Brown to black urine can signal something serious has happened. The dark color can be due to blood cell damage releasing hemoglobin, muscle damage from trauma, or toxins such as onions, garlic, zinc, or acetaminophen (TylenolÂ®).