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.
Black labs are part of a larger group of dogs called Labrador Retrievers, which are originally from Newfoundland. These dogs are popular and make good dogs for active families. An average lifespan for the black lab is 10 to 12 years. Some factors play a large role in determining how long the animal lives, including common diseases and issues among black labs. According to PetMD, common health conditions among labs include patellar luxation, osteochondritis dissecans, or canine elbow and shoulder dysplasia, and canine hip dysplasia.