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.
Dog appears black at a distance, but is not a true black because of the light undercoat. GRAY AND WHITE: gray guard hairs with a light gray, cream, or white undercoat. No red factor is evident. Dog definitely appears gray, even though there may be some black hairs on the topline.RED AND WHITE: a definite shade of red, either light or dark; with light points (lip line and nose), and eye color.og appears black at a distance, but is not a true black because of the light undercoat. GRAY AND WHITE: gray guard hairs with a light gray, cream, or white undercoat. No red factor is evident. Dog definitely appears gray, even though there may be some black hairs on the topline.