Darkness must be the one word equivalent to light in its richness as a spiritual metaphor. Darkness connotes despair, brooding, hopelessness, loss, fear and depression. Light, conversely, is understood as joy, hope, strength, courage and righteousness. Further, darkness can be ignorance, with light as understanding - enlightenment, Light is understood as a spiritual life and pathway; "Torah is light, and the lamp is mitzvah," whereas darkness is being cast into the world without any sense or pursuit of ultimate meaning. Taken in these ways, it's easy to understand the ninth plague of darkness as one when Egypt was forced to face the terrible lack of truth or true spirituality in its way of life and beliefs, so much so that one person could not see the next in such darkness - they could not recognize the light of the Divine, the holiness of each other. "Israel had light in their dwellings;" their true belief in G-d let the light from themselves, their dwellings for the presence of the Divine, shine forth.
There is another way to understand this plague of darkness. The Kedushat Levi writes that G-d actually removed all the barriers darkening his presence, removed the darkness separating us from an awareness of Divine reality. This ensuing light was blinding for the Egyptians, and threw them into darkness again. The Jews were able to perceive this intense light, having the vessels of Torah and belief in one G-d. Here the two interpretations of the plague of darkness converge, one of darkness as spiritual ignorance and untruth, the other as the inability to contain and appreciate Divine light and to be blinded by it. The convergence is with the "dwellings" of the Israelites - in both understandings, it is the vessels we create and utilize which allow us on one hand to create light, and on the other to receive light.
Meditation: Sit up straight and comfortably in your seat. Breathe deeply and naturally a few times, following the natural in-flow and out-flow of the breath. Keep your eyes closed and focus on the back of your eyelids, the dark red, and notice the random images that appear there. Keep aware of your breath, and ride it slowly into a deeper place of quiet, a darker place of yourself. Go under your surface thoughts and images arising in your mind's eye. Follow each breath gently and steadfastly to a deeper level, feeling the space in your abdomen and in your mind. Do not follow the surface waves on the ocean of your consciousness; look under to the open calm. Open your heart and mind to this space of yourself, dwell in its infinite spaciousness. Be a dwelling for divine light, and a window for it to the world. Breathe.