Well, the attraction of the magnet produces a force perpendicular to the blackboard, which is rough, thus producing a friction force to balance gravity, which is equivalent to holding it in one hand. When you put a magnet on the blackboard, the magnet attracts the blackboard by its magnetic force, and then the magnet tends to fall down. However, due to the strong force between the magnet and the blackboard and the rough blackboard, the magnet will be subjected to an upward friction. When this friction force is greater than or equal to the gravity of the magnet itself, the magnet will not fall off.