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.
Brindle Colored Dogs. Boxers are among the breeds with the brindle gene. Brindles are dogs of a different color. Because brindling mixes black hairs in areas of lighter color -- usually brown or fawn -- brindle-colored dogs look a bit like canine tigers. There's also reverse brindling, in which the black coloration overwhelms the brown, so the dog's coat appears black with brown striping.