{Primal Huntbeast} + {Primal Visitation} +
{Titanic Growth}(x2) vs {Blightsteel Colossus} would send it to the graveyard (technically into its owners library) -- Correct?
Incorrect. As the ability 702.12b (http://imtgapp.com/forum/index.php?action=imtg;area=rule;number=702.12b) Any damage that would be considered lethal does not affect the object.
----the guy that answered is the guy I'm talking about who had Blightsteel. Can someone please justify this?
Since Blightsteel is indestructible it would not be sent to the graveyard.
702.12b (http://imtgapp.com/forum/index.php?action=imtg;area=rule;number=702.12b): A permanent with indestructible can't be destroyed. Such permanents aren't destroyed by lethal damage, and they ignore the state-based action that checks for lethal damage (see rule 704.5g).