A blackboard was a flat vertical surface used for writing on with chalk.
20th century rocket scientist Robert H. Goddard used a blackboard for his profession. (Star Trek: Enterprise, opening credits)
A 1960s classroom on Earth Two contained a blackboard. (TOS: "Miri")
Professor James Moriarty's lair included a blackboard. (TNG: "Elementary, Dear Data")
In 2367, Noonian Soong's laboratory had a green blackboard. (TNG: "Brothers")
Later that year, to Geordi La Forge's amazement, a hologram of Albert Einstein wrote equations on a blackboard as Reginald Barclay looked on and offered suggestions. (TNG: "The Nth Degree")
In 2375, a chalk-drawn plan of Vic's lounge was used to plan the elimination of Frankie Eyes and his henchmen. (DS9: "Badda-Bing, Badda-Bang")