So when you finish the quest line of the Dark Brotherhood the journals of the Black dragon will be displayed in the Sanctuary.
When reading one of the journals you activate the voice line of the Black Dragon like if you were doing the quest, making her say the lines she was suppose to say.
( ノ ゚ー゚)ノ My Copying Skills Are Great ヽ(゚ー゚ヽ)
Greetings from the advice Horker