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.
Thread: The Black and White dog breeds. The Black and White dog breeds Well, I'm bored agian. At first I thought I'd do black and tan but there are hundreds of those breeds! Anyway, this thread is the black and white dog breeds....with, as usual, pics.