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.
The dog in this picture has somewhat black spots that are flush (on the same level) with its skin, with no signs of swelling, crusting, and fluid. They are likely the result of hyper-pigmentation from exposure to the sun. Below are a few more photos showing dogs with black spots on their skin and/or fur: