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.
The Newfoundland dog is a large working dog. Newfoundland dogs can be black, brown or white and black (called Landseer) or gray. However, in Canada, the country of their origin, the only correct colours are black or white and black (Landseer).They were originally bred and used as a working dog for fishermen in the Dominion of Newfoundland (which is now part of Canada).owever, in Canada, the country of their origin, the only correct colours are black or white and black (Landseer). They were originally bred and used as a working dog for fishermen in the Dominion of Newfoundland (which is now part of Canada).