St. John left us a sketch to summarize his whole doctrine. The sketch is of Mount Carmel itself. To climb the mount, he drew up paths. Along these paths are both spiritual and natural goods one encounters on the road to life. But all these paths lead to nowhere except one. He drew a narrow path right in the middle of the sketch leading one straight to the summit of the mountain. He wrote "and on this path is NOTHING (nada), NOTHING (nada), NOTHING (nada), and still at the summit NOTHING!"
In the Ascent of Mount Carmel , Chapter II and III, St. John wrote why he used the analogy of darkness to explain his doctrine. First reason, the way to God is dark. Our journey is illumined by faith alone (or should be illumined). Second, the experience of God is darkness to the senses. The purification and purgation necessary for transformation are experienced as darkness by our senses. They are painful to us because they imply detachment and withdrawal from things not purely for God's glory. When God begins to withdraw the soul from the state of beginners to the state of proficients, the sensible part of the soul experiences difficulties and confusion because God is now strengthening the spirit. This is experienced as pure darkness. Third reason, God as the goal of our journey, is Himself darkness to our intellect. We cannot fully comprehend God or the ways of God. Our intellect is finite and cannot possible grasp something infinite. That is why you hear about experiences of Blessed Teresa of Calcutta and St.Therese and many other holy souls. Everything is experienced as pure darkness, the objects of faith and hope are now cast into the dark and cannot anymore shed light to the intellect. At this point, only the WILL to believe, to hope and to love remains.