Michelle Obama was a vision in blue at last night's State Dinner honoring Mexico. The First Lady debuted a shimmering one-shoulder colbalt gown by American designer Peter Soronen, made of three layers of chiffon, lamé chiffon and silk. (This is the fourth gown by Soronen for Mrs. O: She wore his sparkling pewter gown at the White House's annual Governors Ball, a lavender chiffon gown at the Kennedy Center Honors and a simple black gown at the Time 100 Awards.) "I love the structured bodice that I do for her. I put it in a lot of dresses," Soronen told the AP. "And then I drape and pin and stitch all that lightweight chiffon, and I love that look on her." The First Lady accessorized her look with platinum diamond and sapphire earrings and pewter studded strappy sandals.