Black Dogs, as the name suggests, is entirely black, both fur and skin. I have so far only seen short haired ones.
Their eyes come in a variety of colors, and they glow. Mine has yellow eyes. I have also seen white, red, and icy blue eyes, so I'm sure there's more.
They look like big dogs, the size of a Mastiff or a Great Dane but muscle bound, barrel chested.
Associations
They're associated with the Greek Underworld and mostly Hecate. You don't need to have a relationship with her to have one. I don't, not really.
They're more connected to water due to the River Styx, and of course shadows and all around darkness, spirits, and death.
While I personally would say they're Dark Arts, it's more from their appearance and what they're associated with rather than their temperament and personalities. So they'd be perfect for anyone starting out, whether in general to spirit keeping, or in DA.
Black Dogs are very mellow, some might have more of an attitude than others, but in a quiet sense. They're not very affectionate, I've had one lick me, but other than that they're more akin to cats. They like to be around you and maybe lean against you.
What they're good for.
Obviously, because they're a dog they have an affinity towards protection. They may be mellow, but they do their job well. All they need is to appear and a well placed growl. Mine patrols around my house then lays in the backyard for a while before coming back to me.
They could also help mediums, going to the Underworld and fetching any spirits you want to talk to.
They're also a great start in working on your clairaudience from my experience. Their softer dog sounds seem to be easier to hear. A sigh and a rumble, their claws tapping on the floor as they walk.
Could even get one just to have as a friend. A big buddy that follows you around.
Offerings/Treats
Meat, Bones, River Water, Onyx, Obsidian, any other black gems and rocks, basically anything that isn't food or drink that you'd offer to Underworld deities.
They'd also like running water. Like getting a little fountain, or putting them next to a fish tank, setting them next to your bathtub as you fill it. Or even just pouring a cup of water into another cup a few times. It's more the sound that they like, so you could even just play River sounds, bonus points of it's a River Cave.