I think maybe a bench labrador might fit your requirements. IE a lab meant for show, not work.
They're bulky, black and they can be amazing watch dogs.
They're not guardy per se, but you won't have to worry as much about who they're going to get a long with.
Newfies are another suggestion except for THE HAIR.
Both breeds are actually incredibly sweet dogs, and sometimes just having a bigger dog is enough to deter someone from wanting to break in.
Probably not what you were wanting to hear and I acknowledge that, but just thought I'd give some out-ofo-the-box suggestions for you.