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.
A black dog is the name given to a being found primarily in the folklores of the British Isles. The black dog is essentially a nocturnal apparition, often said to be associated with the Devil or a Hellhound.Its appearance was regarded as a portent of death. One, Tchico (Tchi-coh two Norman words for dog, whence cur), is headless, and is supposed to be the phantom of a past Bailiff of Guernsey, Gaultier de la Salle, who was hanged for falsely accusing one of his vassals. 2  The other dog is known as Bodu or tchen Bodu (tchen being dog in Dgernesiais).  dgÃ¨rnÃ©siais