Blackboards can simply be a piece of board painted with matt black paint, but more modern versions are made from a looped sheet of plastic on rollers, so it can be scrolled to reveal more blank writing space.
Blackboards are often used in for teaching. The chalk marks can be quickly and easily wiped off with a damp cloth, or a special blackboard rubber, a block of wood with a felt pad.
Sticks of processed chalk are made especially for use with blackboards, in white and also in various colours.
Blackboards have disadvantages: they produce a fair amount of dust, depending on the quality of chalk used. Some people find this uncomfortable, or may be allergic to it. However, other methods of displaying information are more expensive and have drawbacks of their own.