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.
Colorblind indicates that the dog sees no color, which is not true. A person who is completely colorblind can only see shades of gray, similar to watching a movie in black and white, but your dog can see some colors.