Chalkboards were historically black with a white tint.
They were made with either:
- slate,
- wood painted with egg whites mixed with charred potatoes (in rural areas),
- wood painted with commercially-made porcelain based ink
This earned them the title blackboards, which used to be true to their name as they were in fact, black.
However in the 20th century, some companies started added green, porcelain enameled paint into a steel base.
This variation is what teachers preferred as it cut down on glare.
Scientifically, green is also the color that is the most noticeable to humans/most detectable.
It also masked the chalk powder when erased in the surface.
While the “greenboard” had slowly replaced their black counterparts, their name apparently didn’t stood out.
Therefore, the name blackboard still remained to the present day (although, the chalkboard is an alternative word introduced instead.)