The narrator uses a simile that is laden with verbal irony to introduce the character of Blanche Ingram in Chapter 17:
Blanche and Mary were of equal stature – straight and tall as poplars. Mary was too slim for her height, but Blanche was moulded like a Dian.
Jane idealizes Blanche's image by comparing her to a statue of Diana and a poplar tree. Diana, the Greco-Roman goddess, represents perfect, desirable femininity. The poplar tree is native to northern climates, including England. Poplar flowers fruit into fluffy white orbs that look like cotton balls all over the tree. Jane is imagining Blanche as a symbol of white, English femininity. Blanche's very name means "white," which on the surface contrasts with Bertha's association with Blackness. It certainly contrasts with the gothic darkness of Thornfield.
Blanche's name and the narrator's language here are ironic because Blanche in fact represents further enmeshment in England's colonial economy. Although Jane and the reader don't yet know the extent of the problem, the darkness of Thornfield is an effect of its colonial entanglement. (The novel operates on racist ideology that maps sin and corruption onto darkness and Blackness.) As a fashionable woman, Blanche consumes many products that are connected with slavery and colonialism. This kind of unethical consumerism had long been held against wealthy women, and the novel holds it against Blanche. Jane, who is not in the class of women who can afford luxury imports, finds Blanche shallow and materialistic. Her name and her description as a poplar or a Diana do not match her substance, which is implicated in all of England's problems. As the novel demonstrates by the end, it is not Blanche who stands to "whiten" Rochester's soul by marrying him, but rather Jane.