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.
Terriers and hounds. Tan dog with a black saddle and white markings (to any extent). Trim. Various-general term. A small amount of white on the chest, muzzle, toes and/or tail tip. Trindle. Various-general term. Brindle tricolour (i.e. black with brindle points and white markings.