Rita has opened the blue box. Note the 'staircase' shape of the inside top of the box. Swiss Jungian psychologist Marie-Louise von Franz has said, "Such round or square structures usually symbolize the Self, to which the ego must submit to fulfill the process of individuation."[a] Note that when the box is closed, the staircase structure is turned upside down, or inverted, indicating inversion of the Self. In Part 1 ("The Sexual Aberrations") of his Three Essays on the Theory of Sexuality (1905), Sigmund Freud referred to the practice of homosexuality as "inversion."
According to Wikipedia, the Self in Jungian theory is one of the archetypes. It signifies the coherent whole, unified consciousness and unconscious of a person - "the totality of the psyche."[b] The Self, according to Jung, is realized as the product of individuation, which in Jungian view is the process of integrating one's personality. For Jung, the Self is symbolised by the circle (especially when divided in four quadrants), the square, or the mandala.
What distinguishes Jungian psychology is the idea that there are two centers of the personality. The ego is the center of consciousness, whereas the Self is the center of the total personality, which includes consciousness, the unconscious, and the ego. The Self is both the whole and the center. While the ego is a self-contained little circle off the center contained within the whole, the Self can be understood as the greater circle.[c]
In our movie, Betty represents Diane's ego and Rita her unconscious. The fact that Betty seems to have disappeared while Rita's opening the blue box, symbolizes the ego submitting to the Self, as mentioned above. The circle near the outside edge of the top of the box (see screencap above) symbolizes the aforementioned off-center circle of the ego. The box as a whole acts as the 'container' for Diane's Self, the totality of her psyche.
a. Man and His Symbols. Ed. with introduction Carl G. Jung. London: Aldus Books, 1964. p. 166.
b. Josepf L. Henderson, "Ancient Myths and Modern Man" in Man and His Symbols, p. 120.
c. Wikipedia, 'Self in Jungian psychology'. Web, n.d. URL = https://en.wikipedia.org/wiki/Self_in_Jungian_psychology.