Geraldine, the force of evil in the poem, is associated with darkness and false light.
When Christabel, who symbolizes innocence, first meets Geraldine (evil), it is night. Christabel encounters Geraldine under the dark trees of the forest; the forest at night is traditionally associated with the devil. Even the moonlight that is mentioned cannot fully penetrate the dark and densely-packed trees of the forest. As we learn early on in the poem, whatever light the moon provides "looks both small and dull." Further, when the two return to Christabel's castle, "not a moonbeam enters," suggesting that Geraldine's evil engulfs the castle.
However, to complicate the good and evil dynamic of the poem, Geraldine, though associated with night and darkness, comes disguised as an angel of light, which follows the Biblical wisdom in 2 Corinthians 11:13-15:
For such men are false apostles, deceitful workmen, disguising themselves as apostles of Christ. And no wonder, for even Satan disguises himself as an angel of light. So it is no surprise if his servants, also, disguise themselves as servants of righteousness. Their end will correspond to their deeds.
Geraldine is described as follows:
Drest in a silken robe of white, That shadowy in the moonlight shone: / The neck that made that white robe wan, Her stately neck, and arms were bare; / Her blue-veined feet unsandal'd were, / And wildly glittered here and there / The gems entangled in her hair. I guess, 'twas frightful there to see / A lady so richly clad as she
While the imagery of "white," "shone," and "glittering" might be associated with light, these images are undermined by the idea that the robe was "wan" or pale in an unhealthy way, while the glittering is wild, not serene, and Geraldine's looks are "frightful."
Christabel invites Geraldine back to her castle. The poem is unfinished, but the invitation is a mistake. When the two are ready to go to bed side by side, Christabel sees that Geraldine has an ominous mark:
Behold! her bosom and half her side— / A sight to dream of, not to tell!
Further, Geraldine manages to put Sir Leoline, Christabel's father, under a spell.
Geraldine's light is a false light: she is better associated with the darkness of the night and the forest in which she first appears to the innocent, prayerful Christabel.