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 bears are smaller than Grizzly or Polar bears. black bear n noun: Refers to person, place, thing, quality, etc. (Asian mammal) ours noir nm nom masculin: s'utilise avec les articles le, l' (devant une voyelle ou un h muet), un. Ex : garÃ§on - nm > On dira le garÃ§on ou un garÃ§on.