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.
My dog is a Blue PitBull. He is such a sweet and smart and beautiful black dog with a white strike on his forehead like a lightning bolt down his face and chest. His Black fur is so soft and shiny. Searched for puppy names and ONYX-A BLACK GEM was perfect.