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.
Black Last Name Meaning. 1. Scottish and English: from Middle English blak(e) âblackâ (Old English blÃ¦c, blaca), a nickname given from the earliest times to a swarthy or darkhaired man. 2.