This refers to the unfinished poem Christabel, by Samuel Taylor Coleridge. The maiden Christabel is praying in a wood when she meets a woman called Geraldine who says that she has been abducted from her home by five men. She asks for Christabel's help and together they make their way back to the castle where Christabel lives with her father, Sir Leoline. Geraldine shares Christabel's bed, and seems to cast an enchantment over her. On meeting Geraldine the next morning, Sir Leoline realises that she is the daughter of his childhood friend, and vows to return her safely to her father. He asks his bard to travel to her father and tell him of his daughter's rescue, but the bard recounts a dream he had, where a dove (Christabel) was strangled by a snake (Geraldine) and asks to go to the wood where Christabel prays and clear it of anything 'unholy'. Geraldine looks at Christabel with a 'serpent's eye' full of 'malice', and Christabel realises that, despite her beauty, Geraldine is malevolent. However, Christabel is unable to warn her father because of the enchantment upon her, and can only beg him to send Geraldine away. Sir Leoline, apparently under a similar spell, takes Geraldine's side and turns away from his daughter.
The scene of Blanche's painting is taken from lines near the end of the poem: "And when the trance was o'er, the maid / Paused awhile, and inly prayed: / Then falling at the Baron's feet, / 'By my mother's soul do I entreat / That thou this woman send away!' / She said: and more she could not say; / For what she knew she could not tell, / O'er-mastered by the mighty spell. / Why is thy cheek so wan and wild, / Sir Leoline? Thy only child / Lies at thy feet, thy joy, thy pride. / So fair, so innocent, so mild;
This might seem like a strange subject for Blanche to paint, using Christabel's namesake as her model, but the poem has undertones of lesbian sexuality that many criticis have noted, so perhaps this is a reflection (by Blanche or by Byatt) of the ambiguous relationship between Blanche Glover and Christabel LaMotte.
You can read the full poem here.