Hello Peter! Try using Black Velour and stroking fine, light strokes with Greys and Whites, and perhaps other light tints to depict the glossy coat of a black doggy.
I have a pastel portrait of a black cat on Black Velour and it looks stunning, if you keep the fine strokes to just the light catching areas on the dogs coat.
Alternatively, try using Pastel Pencils as they may give you better control in the fine hair depiction, and as we always say; Practice, Practice. Practice!!!
I hope this may offer some help, but come back to me if you wish.
Very best wishes,
RSC Wigan / Warrington area