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.
dark color means the dog is concentrating it's urine. Could be something as simple as not drinking enough water that day to something as serious as acute kidney failure.ark color means the dog is concentrating it's urine. Could be something as simple as not drinking enough water that day to something as serious as acute kidney failure.