My opponent has to sacrifice a creature. He sacrifices a Blightsteel Colossus. I have Grave Betrayal on the battlefield. Will my opponent get to shuffle the colossus into his deck again, or will it return to the battlefield under my control?
Consider this portion of the text on Blightsteel Colossus.
This text creates a replacement effect. A replacement effect "watches" for an event to happen. For example, regeneration is a replacement effect that watches for the creature to be destroyed.
The rules for replacement effects neatly state that the event being replaced never happens.
So, when Blightsteel Colossus starts to head for the graveyard, instead, he is revealed and shuffled into his owner's library. The event that got replaced never happens.
To die in Magic has a specific meaning:
Blightsteel Colossus never went to the graveyard, and so Blightsteel Colossus never died. Therefore, Grave Betrayal never triggers.
It will be shuffled into his deck.
To die is to be moved from the battlefield to the graveyard[CR 700.4]. Blightsteel Colossus never dies because he moves to its owner's library instead of moving to the graveyard when he is sacrificed[CR 614.6]. As such, Grave Betrayal's ability never triggers.
It never goes to the graveyard and so it does not get affected by the delayed triggered event of Grave Betrayal.
They get to shuffle it back into their deck.