William Black, Abby’s husband, is the “Black” of the envelope that the key was inside; or, to be more precise, William’s father was the “Black” of the envelope, because the key opens his father’s safety deposit box. When William’s father was dying, he wrote letters to all of his friends and acquaintances. The letter he wrote to William says that there’s a key to the father’s safety deposit box inside a blue vase, but before William had read the letter, he sold that blue vase in an estate sale to Oskar’s Dad. William had searched for Oskar’s Dad, but since it was right after September 11, the search went nowhere. William had a complicated relationship with his father and never got to say goodbye, so finding the key helps provide some closure for him.