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.
Coat color and pattern is a polygenic trait, meaning it is controlled by more than one gene or gene complex.A dog with the gene for black color expression may also have the gene for suppression of black and so have a red or yellow coat. Polygenic traits are [characteristics of organisms that are] â¦  influenced by several genes.    So multiple alleles are more than two alleles for one trait, and  polygenic traits are one trait that is influenced by mulitple  genes.