Meta on Sam's powers & Demon blood
Samuel Winchester was born with an unknown psychic Potential. The Demon Azazel - perhaps having caught wind of the Future Prophet memo ten years before - had already scouted out the most likely parents-to-be for permission to enter their homes at a later date (possibly bypassing a looming Archangel's threat meter).
A child's Potential is full-formed, but undefined at six months of age. Untouched it may become telepathy, aura or imprint reading, pyrokinetics, mind control, telekinesis, visions, or any number of other things that could prove dangerous to the Demon's plans (whatever they may be), so he uses his blood to paralyze the child psychic's "muscle" for the next twenty some-odd years.
In the interim the infected child will grow up sensing something greater, something special within that he or she cannot access. The (likely subconscious) frustration will cause the child to either act out or shut down, alienating him/her from his/her strongest supports, and fully abandon them in a new world when the paralyzed muscle finally begins to spasm awake.
But the demon blood isn't quite finished: despite the apparent weakness, it still holds enough power to display this now-adult's next-strongest ability in a single emotion-charged event that will likely leave him/her vulnerable and frightened - ready for a Yellow-Eyed Man's friendly offer to mentor them in Cold Oak, South Dakota . . .
This is the base plan. But Sam Winchester grew up learning to fight and kill spirits, monsters, and demons, so he - and any psykids who survived meeting him - would require a more . . . delicate touch.
Though Sam had left his hunting family, he'd already started building one of his own at Stanford when the dreams first appeared, and, despite the devastation of Jess' loss, was still not isolated enough to safely approach. The Meg-demon's attempt at seduction (was that really Dad on the other end of those coordinates?) in Scarecrow failed to pull Sam any further away. Had Dean died or abandoned him after Sam's telekinetic surge in Nightmare, the plan might've gotten back on schedule. Trying to sicc Daevas on the elder Winchesters only reunited the family and killed the Meg-demon's meatsuit (Shadow). All an interesting annoyance, but still not that big a deal . . . until the Winchesters got ahold of the Colt.
An act of War. Azazel was already busy seeding the next generation of psykids and perhaps couldn't risk the distraction, so he sent his two demon-kids off to grab the Winchesters' attention and the Colt, only to be interrupted in Salvation. Well, at least his idiot kids had managed to hold onto John.
Events of Devil's Trap and In My Time of Dying take place, and the boys still manage to slip through his fingers. Without the blood connection, Azazel has no way to track Sam, and time is growing short. He can't risk a rogue visionary informing and gathering his kids against him, so he sets up Croatoan to once again paralyze Sam's third eye indefinitely.
While rescuing Sam in Hunted, Ava Wilson learns just enough details to become a threat herself and gets bumped up into the contest at Cold Oak. She manages to survive until All Hell Breaks Loose, wherein the most promising survivors must fight or die. Then in comes Dean (& Co) to flip the tables yet again, and Azazel dies as the Devil's Gate opens.
Thirteen months later Dean's dead, Sam's finally ready to follow the plan, and in slinks a demon called Ruby with a solution that could destroy the world . . .
Will Sam's visions ever return? Can they, with Ruby's blood flowing through him? Only Kripke knows.