But if you do get a tail for him, you could get a medium grey or something. Black would look harsh and fake with his natural tail unless you dye it. Black would be cheaper though, so it might be worth it to dye his tail.
If his mane and tail are mostly black, you should defiantly go with black. Black is usually very inexpensive. But if the tail isn't totally black, I would dye it. Just get black hair dye from your local store and dye his tail with it!