Noob here but from what I could find you'd create an event handler that listens for destroyed blocks using a handleBlockDestruction method.
You'd then check if the event.entity has the correct pickaxe equipped. If so, you can iterate through the affected blocks. Should be all blocks with either x, y or z coordinate one greater or smaller than the destoryed one.
Not sure about how to add the actual item to the game. For wireframing you can try to have all pickaxes affected at first. But you'll probably find some tutorials on how to create new items.