These guys usually spawn in Salisbury Plains near the Tomb Raider circles. It is not true at all that these mobs spawn only at night because I have seen several during the day.
I was running through Salisbury Plains near the tower by Camp Forest(at night) when I was suddenly jumped by a level 11 black dog! I'm level 10 and it was orange to me. But the guards at the tower saved my butt =)
I ran around all night once and still couldn't find a black dog, any other places they might be found? I need to find one for a quest, and I did look at night but after running around and not finding one I gave up after it became day again. Maybe I just have bad luck but, just couldn't seem to spot one.
I had trouble finding the black dog also but finally after the task time ran out I found one. I t was located in Salisbury Plains NE of the tower leading to Camp Forest. It was walking near the road. I know this info is too late for you but maybe it will help someone else searching.