Symbolism, Imagery, Allegory
Just the name alone gives us the heebie-jeebies. The dark room is where Mama Elena bathes and is the only place she is naked and vulnerable.
Only Tita, whose mission it was to serve her until death, was allowed to be present during this ritual, to see her mother naked. (5,327)
Why is it such a big deal for her to be in her birthday suit? For her, the body and anything remotely sexual causes her pain. Her love and stud muffin, Jose, was killed suddenly, and that was her one chance at happiness.
On the flipside, when Pedro and Tita have sex for the first time in the dark room, it is transformed to a place of glowing light:
Plumes of phosphorescent colors were ascending to the sky like delicate Bengal lights. (8, 538)
For the two love birds, the dark room is a place to be free and safe from the eyes of the rest of the ranch. As Bob Marley so eloquently crooned, "in the darkness there must come out the light."
And, as Dr. Brown told Tita, "[…] each of us is born with a box of matches inside of us but we can't strike them all by ourselves" […]. What they were both saying is that even the darkest of places can be lit up, ignited, sparked, excited. Even the dark room. Even the dark depths of Mama Elena's soul.