Is Blanche crazy? Or what is her sickness?
Blanche is a romanticist. This is due to a number of influences. She was raised like a Southern belle in a typical Southern mansion called Belle Reve (which means beautiful dream). Girls of that era and social class spent much of their time reading, and characteristically they liked romantic poetry and romantic novels. If they went to school it would be to a young ladies finishing school where they were taught nothing but impractical subjects such as music, art, literature, and etiquette. Refinement without money is hard to sustain. She tells how her father and other male relatives wasted money and had to sell off all the land around the mansion, so that it was inevitable that the building itself would deteriorate and go into foreclosure. Forced to do something to earn her own living, Blanche became a teacher, and she naturally taught the romantic subjects she knew, such as the works of authors like Lord Byron and Sir Walter Scott. Her interest in young boys is consistent with her romantic character. She is not attracted to the carnal aspects of sex but to the romantic aspects. She likes boys who look beautiful and sensitive. She was married to such a youth, but he committed suicide. He may have been too romantic and sensitive to succeed at anything in the harsh world of reality, especially in a region where the economy was ruined by war.
Blanche herself is struggling to survive in that cold, cruel world. She, of course, symbolizes the Old South which was dealt a death blow by the Civil War and is being overwhelmed by strong, brutal, aggressive, selfish, immoral realists like Stanley Kowalski. William Faulkner dwells on a similar theme in many of his great novels, notably in the "Snopes Trilogy," consisting of The Hamlet, The Town, and The Mansion. Blanche only moves in with her sister and brother-in-law out of desperation. She has run out of alternatives. She is broke and probably can't pursue a career as a teacher because she has destroyed her reputation by immoral conduct. This immoral conduct is indicative of the decadence of the Old South and was exhibited by the male relatives who were responsible for the loss of Belle Reve.
Stanley understands her immediately. He would classify her as a phoney. He knows she despises him, and therefore he enjoys tearing her illusions apart. Without her illusions she loses her mind and has to be institutionalized. Tennessee Williams has done such a marvelous job of dramatizing the changing social conditions of the Old South that his play hardly needs any explication. We sympathize with Blanche. She is in some ways a better person than Stanley, but hers is the kind of superiority that depends on income, and the wealth of the Old South depended on the exploitation of slaves.
I think you may be referring to 'A Streetcar Named Desire' so perhaps check there, but it is debatable as to whether she is crazy or not, certainly by the end of the play she is deranged but her 'sickness' is in fact not a sickness at all, Stanley has reduced her to nothing by raping her. Her fragile state of mind is a result of the scarring suicide of her husband, and the decline of Belle Reve, in conjunction with other things.