Why not go to the local shelter and look at some medium to largeish black dogs, I've found no matter what the breed or mix if the dog is black people will make sure to stay out of that dogs way.
Also, if you're up for putting in some exercise time, an Aussie/Aussie mix could be a good dog for you to look at. They really are excellent watchdogs and most are willing to protect their homes/people if it comes right down to it.