Clarification:
Stealthy Stride is a Simple Action. An attack is a Standard Action. Does that mean that in a single turn you can use Stealthy Stride to treat your opponent as blindsided, and then attack them?
Quick answer: No.
Longer answer: Statuses are determined at during the declarations phase. You may attack and then use Stealthy Stride to hide so that your target will be blindsided the next round, but you cannot start your turn without a status, quickly gain that status, and then use it all within the same turn.
Blindsided versus Surprised
A target is surprised when they are not expecting an attack. A target is blindsided when they are unaware of or unable to see their attacker. Being unable to see an attacker does not mean you are not expecting an attack - when you disappear into the bushes, the target is not surprised to see you come back out out of the bushes, even if they could not see precisely where you were!
The one-turn version
Attack, then Stealthy Stride. If the enemy cannot see you, they cannot hit you back!
The two-turn version
Round One: Attack, then Stealthy Stride.
Round Two: The opponent is unable to see you at the declaration phase, making them blindsided against you. Move back into range and Attack. Surprise Strike applies to the attack (if you have it).
The three-turn version
Round One: Attack, then Stealthy Stride.
Round Two: The target is unable to see you at the declaration phase, making them blindsided against you. Remain hidden for the whole turn, moving to a different location than the one in which you were last seen.
Round Three: Move back into range and attack. The target is not expecting an attack from that direction, making them surprised! Ambush or Surprise Strike (but not both simultaneously) applies to the attack (if you have it).