The recent RearView review, and twitter #wfbrewatch of About A Boy reminded me of a potentially major flaw in the logic to the entire Mark of Cain storyline which played out over the second half of S9 and all of S10. What was holding back The Darkness while Teen Dean was grooving to Shake It Off?
In First Born, Cain said the Mark could be transferred to someone worthy. This implies that the Mark is transferred, and not shared with someone else. This is supported by the fact that Dean then killed Cain in Executioner's Song with the First Blade. The First Blade can kill a Knight of Hell, it can't kill someone carrying the Mark of Cain. Death confirmed this in Brother's Keeper when he said Dean could not be killed.
DEATH - "And that Mark on your arm is the First Curse. Nothing can kill you." So, if Cain was still carrying the Mark of Cain, Dean could not have killed him.
This is contradicted by what Cain said in 10.14 The Executioner's Song when he told Castiel "Humans. Eh, the Mark thirsts for all kinds.", implying that Cain still carried the Mark.
Death said in Brother's Keeper - "Lucifer passed the Mark to Cain, who passed the Mark to you, the proverbial finger in the dike."
Death also said - "So I could remove the Mark, but only if you will share it with another… …to ensure that the lock remains unbroken and the Darkness remains banned." This would imply that the Mark is shared, and not passed on.
If the Mark was shared, and not passed along, then Lucifer would still have it and The Darkness would not have been released. So when Dean was turned in to a teenager and no longer had the Mark, what was holding back The Darkness?
When the Mark was removed from Dean by Rowena, this opened the door to The Darkness. So that supports the fact that the Mark is passed on/transferred, and not shared; otherwise Lucifer still had it. And we're back to my original question; what was holding back The Darkness while Dean was turned in to a teenager?