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 Cat Tales, Symbols and Meanings. Black cats are a symbol of the soul's mysterious coming to earthly life. Their impenetrable color represents the enigma of the spirit's return voyage to the physical world. The cat in itself is a powerful symbol of wholeness and unity, a joining of the spiritual with the tangible. Color it black and you get a lot more compelling symbolism. Old folks believed that a cat with black fur was the spirit of a human who died with unfinished business and was restless. Such spirits cannot be born again as humans and may only take possession of this particular inky cat and other black animals when they wish to return to the physical world.