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.
Owners describe their seal dogs as looking black at first, but if standing next to a true black dog, they have faded hairs though out their coats. From the images I have found, I think their nose is black, so that would rule out brown.