Blackboards haven't been black for a half a century now. In the 1960s they were green, and in the early 21st century they've been replaced by white boards and smart boards. But, back a hundred years ago, blackboards were heavy black slate boards that covered the front of a classroom so that a teacher could demonstrate a math concept, write a poem to memorize, or simply act as a reminder to students of classroom rules.
In Oklahoma City recently, a school renovation projected revealed blackboards untouched since 1917! Here's a short news video of the discovery:
The Washington Post also covered the story.
The story of education throughout our history is quite interesting, especially for children who are still in their elementary school days. For more information about education in American history, look for these children's books Going to School in Colonial America by Shelley Swanson Sateren [J 370.9 SAT] and Going to School in Pioneer Times by Kerry A. Graves [J 370.9 GRA], both of which are part of the "Going to school in history" series. Or Don't Whistle in School: The History of America's Public Schools by Ruth Tenzer Feldman [J 370 FEL]. For adults we have School, the Story of American Public Education [370 SCH].