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.
This breed is a medium- to large-sized dog that is named for the black coloration on its muzzle. Black Mouth Curs are often referred to as âmultitalentedâ dogs because they can be trained for a variety of uses including hunting, herding, and general utility â they also make great family pets!