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.
Blue heelers are short, silver colored dogs from Australia. The word blue comes from their coloring, which tends to be a silverish that can look blue in some lights. The ter â¦ m heeler is referring to a class of herding dog breeds that are short, like the blue heeler.