The FAQ says:
1. I'm playing a battlemind. If an adjacent enemy that I have marked attempts to shift to another square adjacent to me, can I use blurred step to shift to that square and prevent the enemy from entering it?
Yes, blurred step is an opportunity action, which interrupts its trigger.
So, you do know where they're shifting and that shift is blocked. If they're doing a 'shift 5' type maneuver, then they still have another 4 squares worth of shift they can do something else with. If you choose to use Blurred Step instead of an opportunity attack to follow someone who is making a move action to just move 5 squares, it is similarly limited. If you don't have a power or melee training to make your opportunity attack notable, this might very much come up.