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.
With that in mind, Mastiffs (OEMs) are generally very large dogs; fawn, apricot or brindle in color; all with a black mask and ears; possessing a medium to short coat with very little white (which, if it appears, should be confined to the chest but often appears on the toes as well).