I'm assuming that your assignment is to write some sort of compare and contrast essay. Without seeing the actual prompt, I can't give you many hints for your thesis, but make sure it directly addresses the question asked in the prompt. Then, use the rest of your introduction to set up what you'll talk about in each of your body paragraphs. Your organization is up to you; however, you may want to focus on one trait in each paragraph and explain how it's different in each of the characters. Or, you may want to use 2 body paragraphs describing Jane, then 2 body paragraphs describing Blanche. Yet if you focus on a trait in each paragraph, you'll have the advantage of contrasting them side-by-side, which may have more impact on your reader.
You've pointed out many characteristics of Blanche, but keep in mind that she is intended to be a foil to Jane: Her emptiness makes Jane's true depths all the more evident and attractive. Blanche is also the physical opposite of Jane. she is very tall but has a proud, arrogant manner, a "mocking air," and a "satirical laugh." She tends to pretend expertise in subjects about which she actually knows nothing, especially when she speaks to her snobbish mother, Lady Ingram. She treats Jane with extreme condescension and exhibits a "spiteful antipathy" toward Adele. Despite all these obvious faults, Jane thinks that Blanche will marry Rochester. Ironically, this apparent certainty makes Jane more passionate toward Rochester, who in turn reveals that he had no intention of marrying Blanche.
Jane, on the other hand, is genuine, thoughtful, and in control of her own life whenever she can be. Physically plain and slight, Jane is acutely intelligent and fiercely independent. She is also a shrewd judge of character. Throughout the novel, she relies on her intelligence and determination to achieve self-fulfillment. Yet her strength of character does not make her immune to suffering; on the contrary, she suffers because she is so keenly aware of the difference between how things are and how they might be. Although she is rebellious when rebellion is called for, she is inherently conscious that actions must be tempered by reason. When she refuses to become Rochester's mistress, she cites a higher moral law as her justification: "Laws and principles are not for the time when there is no temptation; they are for such moments as this, when body and soul rise against their rigor. . . ." In this action, as well as in refusing to marry St. John Rivers, she proves her unwillingness to compromise her principles. Utterly opposed to hypocrisy, she nonetheless is capable of recognizing that goodness exists within flawed human beings. It is this sense which allows her to marry Rochester when they are both morally ready.