Rebecca Blank, who was named last fall the first female president of Northwestern University, announced Monday she is stepping down after being diagnosed with an aggressive form of cancer.In a news release, the school said that Blank, who concluded her eight-year tenure as chancellor at the University of Wisconsin-Madison in May, will return to the Madison area for cancer treatment.“I do not have the words to express to you how disappointed and sad I am to be telling you this,” Blank said in a statement. &ldquo …