LOS ANGELES (Reuters) – Beyonce is letting fans into her private life, introducing daughter Blue Ivy to the world and talking about motherhood, marriage and miscarriage in a new documentary. The 31-year-old pop singer and her rapper husband, Jay-Z, 43, one of music's most influential couples, have guarded their privacy fiercely. But in "Life is But a Dream," airing on Saturday on cable channel HBO, Beyonce gives fans a glimpse of how she works and even a peek at baby Blue Ivy, who has been shielded from paparazzi since her birth in January 2012. …