Blackboards

Though the term blackboard has a color in its name, most of them are now not actually black. We still use the term interchangeably with chalkboards. Now they are more often green.
A few hundred years ago, blackboards were black. They were large boards of connected slates that teachers could write on for the whole class to see. The name blackboard was not used until 1815. They were usually made with slate, but in rural areas, they were often wooden boards painted dark with egg whites mixed with the remains of charred potatoes. Later, they were also made of wood darkened with a commercially made porcelain-based ink.
During the 20th century, manufacturers began to make chalkboards using a green, porcelain enameled paint on a steel base. By the 1960s, the green chalkboard trend was becoming universal. Teachers had discovered that a different colored paint was a lot more comfortable to stare at all day, because green porcelain paint cut down on glare.

During the 1990s, schools began converting their classrooms to whiteboards, which produce less dust and eliminate that telltale screeching noise.