MEXICO CITY — An anonymous, twentysomething blogger is giving Mexicans what they can't get elsewhere — an inside view of their country's raging drug war. Operating from behind a thick curtain of computer security, Blog del Narco in less than six months has become Mexico's go-to Internet site at a time when mainstream media are feeling pressure and threats to stay away from the story. Many postings, including warnings and a beheading, appear to come directly from drug traffickers. Others depict crime scenes accessible only to military or police. The undifferentiated content suggests that all sides are using the blog...