Schwarzbiers are bottom-fermented beers, though originally top-fermenting yeast was used in brewing them. The alcohol content usually ranges from 4.8%–5%. They get their dark color from the use of particularly dark malts in brewing. The malt in turn gets its color during the roasting procedure. Dark beers are some of the original types of beers brewed, with lighter colors only becoming possible with lighter, short-roasted malts approximately one hundred years ago.
I find it fascinating how historical changes like those to barley roasts have led to such a diverse range of beer styles.