Images of darknesss can be found in Act IV scene 1, which is when Juliet comes to Friar Lawrence desperate and suicidal concerning the situation that she is placed in: she is being forced to marry Paris against her wishes, because, unbeknownst to her father, she is already married to Romeo, who has been exiled to Mantua because of his murder of Tybalt. Note what she would rather do instead of being told to marry Tybalt:
...bid me go into a new-made grave
And hide me with a dead man in his tomb.
The darkness of such imagery, combined with her desire to be hid at night in a "charnel house" suggests the violence of Juliet's emotions and the sense of start entrapment that she feels as it appears that she has no escape or way of avoiding the fate of being married to Paris. Any horror, or any darkness, is preferable to this fate, even the most gothic terrors of being buried with a dead corpse in that man's tomb.
Images of light are evident throughout the play, not only when Juliet is described by Romeo as "the sun," but also in the way that images of light are associated with images of darkness when the Nurse discovers that Juliet has supposedly died after taking the potion that Friar Lawrence gave her. Note how she describes the day as being "hateful" and "so black a day" as she has never seen, converting something that would normally symbolise light into something that is representative of darkness.