it signified the guilt Betty Elms/Diane Selwyn would have after ordering the hit on Camilla Rhodes/Rita

remember what the hitman told her "youll know why I gave this to you after the hit has happened" (this was when he gave her the key..something to this effect)

at least, i assumed the blue box (as well as the old people who tormented her) were just physical personifications of the guilt she would have over what she did as well as the remorse and pain she will fill for eternity for killing the ONE person she loved....

Ryan