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.
Blackmouth Cur Dog Breed, Puppies, Grooming and Life Span. The blackmouth cur dog breed is one of the oldest dog breeds that is known to belong to the hunting group and in fact is also considered to be a multipurpose dog breed. The origins of this particular breed of dogs can be traced back to the United States of America.