The main thing with animals is to get down on their level for a more interesting intimate shot. With a black dog you need to avoid under-exposure, so meter carefully and shoot in RAW. I'd normally say bracket your exposures like Hell, but depending how unruly the dog is, you'll be taking your shots when you can get 'em.
Avoid shooting around midday if it's very sunny. The reflectors are a good idea if you can get the dog to keep still in one place long enough to position them, or try fill flash. The owner standing right behind you can help hold the dog's attention.
Good luck.
