Later, a growing industry was able to offer plastic and metal boards to schools. The latter are widely used due to their magnetic properties, which turned out to be extremely convenient in everyday use. Initially, such boards were covered with matte black paint - no one thought about changing the color to another. It was assumed that white traces of chalk would be clearly visible on a black background.
However, science does not stand still - by the middle of the 20th century, experts determined that when working with images for a long time, green is the most optimal color for the eyes. Schoolteachers who received trial batches of blackboards with a dark green finish confirmed that the color worked much better, especially on bright sunny days, while blackboards with white lettering create too much contrast in sunlight.