黒板
こくばん · kokuban · JLPT N3 · noun
blackboard
🏫 黒板 = 黒 black + 板 board, so a black board
黒 (koku) means black, as in 黒い kuroi. 板 (ban) means board or plank, as in まな板 manaita, a cutting board. Put them together and you get a blackboard.
板 — wood radical 木 + 反 (fǎn, phonetic)
板 is a semantic-phonetic compound. The 木 'wood' radical signals the meaning — a flat piece of wood, i.e. a board or plank. The right side 反 supplies the sound, historically close to 'bǎn/pàn'. Over time it extended to mean any flat panel, and figuratively 'stiff' or 'inflexible'.