It all makes sense now.
The chalkboard never existed to begin with. It was a figment of our minds. An illusion. The 'chalkboard' is a metaphysical symbol of our childhood, where we spent all of our lives as children, staring into the chalkboard, letting it dictate our lives. But. The chalkboard is an illusion. Hence, our lives are an illusion.
That is the meaning of the invisible chalkboard.
EDIT: And now it has returned.