|The catch light in the middle of the eye here gives the impression of on camera flash. On a second appraisal I can see that it isn't as the light is a little softer, however appears to still be coming from pretty close to the camera.
I would try moving the light a little more to the left and if you have a reflector either use this to fill the shadows, or try to reflect some light back onto the top of the head. This will help create more separation of the dark fur against the dark background as they seem to be blending together a little.
Black backgrounds are very difficult as they tend to create a vacuum of nothingness, and dark objects against a black background tend to blend in. You are almost better with a white background, which when unlighted will become grey, or a plain coloured background.
I think what also doesn't help this image is the blanket, which gives the impression of a snapshot when the dog was in its basket or bed. As this challenge is defined as Pet Portrait, then in my opinion a more studio like (Interpreted as sterile) background would probably be more fitting.