Blight means ruin and disaster. Words and phrases that indicate and support this meaning are: "the last bitter hour" (denoting death), "stern agony, and shroud, and pall" (also denoting death, but this time in an even more literal way), "breathless darkness", "make thee to shudder, and grow sick at heart" (these phrases definitely imply a disaster that takes our breath away).
Juliet says that marriage is something that does not draw her attention, and then her mother said that she should start thinking about it becuase there are younger mother that are made and have kids