A black key was a piece of evidence in Phoenix Wright's investigation into the murder of Turner Grey. The key itself was for the door to the Channeling Chamber in Kurain Village's Fey Manor.
A locked room mystery[]
Maya Fey used the key to lock the Channeling Chamber with her and Turner Grey inside. Unbeknownst to them, Mimi Miney was hiding in the chamber as part of a plot by Maya's aunt Morgan to kill Grey and frame Maya. After Maya was drugged, Grey was stabbed and then shot to death. Phoenix Wright and Lotta Hart, who were outside the room at the time, heard the gunshot and broke down the door to find Miney dressed identically to Maya, although Morgan directed them to call the police before they could do any detailed investigation. This allowed the co-conspirators Morgan and Miney to hide the key inside Miney's robes and burn them in a nearby incinerator, leading to Pearl Fey later discovering the intact key.
During the first day of Maya's trial for Grey's murder, Wright presented the key in an attempt to argue that Maya had left the chamber before Grey was killed. He argued that it wasn't on her when she was found after the murder. This convinced the judge to extend the trial for another day.