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.
This page provides all possible meanings and translations of the word blackguard. cad, bounder, blackguard, dog, hound, heel(verb) someone who is morally reprehensible. you dirty dog. ridicule, roast, guy, blackguard, laugh at, jest at, rib, make fun, poke fun(verb)