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.
What Does a Growling Dog Mean? As dog owners we should become very good at reading how well our dog is feeling, simply by observing their behaviors. Sometimes we miss the mark, and start to play with our dog resulting in an unexpected growl as a warning to leave them alone, as now is not the time for rough-housing. This is the time we must step back and examine the dog for injuries or illness. It is possible the dog has become injured either while playing or in a fight with another dog. If he or she has been hit by a car the signs will normally be very obvious as will the signs present after a dog fight.