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.
Is a breed of dog known for its distinctive features of deep wrinkles and a blue-black tongue. The breed comes from China. The name (English name probably derived from British spelling of the Cantonese equivalent, sÄ peih) translates to sand skin and refers to the texture of its short, rough coat.