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 term denotes colors the dog is capable of producing. There are 4 Rare Colors in English Bulldogs, they are Lilac, Blue, Chocolate, and Black. QUAD means 4, as in QUAD Runner, QUAD Core, etc. A QUAD dog can produce all 4 of the Rare Colors.