Guinness is a good product, but a bit of an acquired taste. The average American beer drinker is attuned to light pilsners as a result of our hot summers. Guinness is not about to replace them.
Black lager—at least the German kinds I’ve had, is in no way like a stout—except in appearance.
The Koestritzer brand I mentioned above (very hard to find here in the states) is very smooth and light—so much so if you closed your eyes, you wouldn’t think you were drinking a dark beer at all. The advertizing for Koestritzer calls it, “The black beer with the blonde soul.”
Actually, Sam Adams “Black Lager” is a very good product, fairly similar to Koestritzer.