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.
Etymology. The name Sirius Black is a pun on his Animagus form of a black dog since the star Sirius is known as the Dog Star and is the brightest star in Canis Major, the Great Dog constellation. Sirius is derived from the Ancient Greek ÏÎµÎ¯ÏÎ¹Î¿Ï, Seirios, meaning glowing or scorcher..In Greek mythology, it is Orion's dog.tymology. The name Sirius Black is a pun on his Animagus form of a black dog since the star Sirius is known as the Dog Star and is the brightest star in Canis Major, the Great Dog constellation. Sirius is derived from the Ancient Greek ÏÎµÎ¯ÏÎ¹Î¿Ï, Seirios, meaning glowing or scorcher..