Whoops!
What I had assumed previously to be another color variant for rottweilers was in fact not. At least, not exactly. My love of red rottweilers made me want to explore this a bit and I'd thought I'd seen something of the nature of what I took to be a black rott. In point of fact it wasn't a black rott and there may be no such thing.
It turned out to be a rottweiler/black labrador mix.
My bad.