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.
Dog Coat Colours and Patterns. banded hairs; deeper colour at the tip than near the base, and some are black at the tip and have one or more bands of brown, red, yellow or cream colour as you move toward the base of the hair; ticked. a dog that has some shade of black or brown and also white but no tan in breeds like the Shetland Sheepdog. this colour pattern generally follows the same pattern as the Salt and Pepper (see below), however the entire Salt and Pepper section must be black; as in Schnauzers.