I also tamed one of those and from what I've seen it comes in two colours. One is red, the other is a dark grey that may or may not appear black depending on lighting. I've taken a few pictures to illustrate this.
First in the wild, selected and not selected:
This is the same Gilnean Hound in the same area. I used selected/not selected to illustrate the effects of different lighting without the risk of accidentally taking pictures of two different skins (since there are skins in game that are very subtelly different and the lighting in Gilneas is really bad).
Now two pictures of the tamed pet in Stormwind. Same area, same time of day, but in the second picture the light comes from behind the Mastiff, making it look black(er).
The Mastiff kind of looks like it has a different colour in each picture. Since this skin is so dependent on lighting I guess even something like your settings can make a difference in what you see, as well as what areas you're in, which is probably what's causing the confusion. But I think that this dark grey skin is indeed the black skin because the pictures of the officially grey skin I've seen look much brighter.
And after running around the Ruins of Gilneas for some time and countless resummons I'm pretty sure that it only comes in black and red (and it greatly prefers black as it's hardly ever red - in the wild or as a pet).
And just for completion two pics of the red skin:
I hope this helps.