Of course, it's a black lager. (Lagers don't have to be light; you didn't think Guinness would make a light lager, did you?) Technically, Guinness Black Lager is a schwarzbier, though Guinness -- being the national symbol of Ireland and all -- isn't going to go around advertising that it's making a German-style beer.
So what's Guinness Black Lager like? Well, it's quite a bit different from Guinness' stouts, even if it looks sort of similiar.
Cola brown with a tan head that is probably best described as bumpy, the beer has a sweet aroma that suggests dark fruit and sugar. This lager -- which should be served cold, unlike stouts -- tastes slightly sweet, too, and nutty with caramel overtones. It's a bit thinner than other schwarzbiers, though, and considerably less substantial than Guinness' other beers (especially its best one, Guinness Foreign Extra Stout). On the other hand, the alcohol content is fairly tame -- only about 4.5 percent, which is a bit lower than the average schwarzbier.
My verdict: It's OK, but there are better schwarzbiers out there, including Samuel Adams Black Lager.