What is Black bread as refered to in Russian literature?

It would be rye bread probably made from rye flour alone. Rye flour is quite brown, darker even than whole wheat flour, and it doesn't rise as high as wheat flour so the loaves tend to be heavy or dense and on the damp side, and dark coloured. It would have been what the poorer people ate, the ones who couldn't afford wheat or more refined flour. Comparable to a pumpernickel, perhaps, though the pumpernickel you get in North America is often coloured to be very dark. Most of the rye bread you get in North America doesn't have a very high proportion of rye flour so it's very light when compared to an all rye loaf.

Shwarzbrot. Dark rye bread.

Black bread is made with rye rather than wheat.

Ask any search machine:

"black bread" dictionary

http://www.google.com.ua/search?num=100&hl=ru&q=%2...

You’ll get a lot of answers.

A kind of very dark coarse rye bread, pumpernickel

http://www.thefreedictionary.com/black+bread

A coarse-grained dark bread, often sour and made from whole-grain rye flour, pumpernickel.

http://dictionary.reference.com/browse/black+bread...

A coarse wholemeal wheat or rye bread leavened with sourdough.

http://www.answers.com/topic/bread