Question goes as titled.
Say I attack, opponent blocks, is there a time I can cast Distortion Strike thus making my creature unblockable?
Distortion Strike is a sorcery. You can only cast it during the pre-combat main phase or the post-combat main phase.
I have no idea why you'd want to do it post-combat, but you could.
I am going to say no if only because Distortion Strike is a sorcery and not an instant and only instants can be cast during the attack step.
Even if you used an instant like Infiltrate , waiting until after blockers are declared is too late. A creature being unblockable means creatures can't be declared to block it. Once a blocker is declared, the attacking creature stays blocked for the rest of the Combat phase, regardless of what happens to it or the blocking creature after that point.
No. Even if you had Leyline of Anticipation , which would allow you to flash-cast Distortion Strike , you have to cast it before blockers. Unblockable means that nothing can be assigned a blocker to that particular creature. If a creature gains the ability after it has already been blocked, the ability is effectively disregarded for that instance of combat.
If you cast something like World at War in the second main phase, then the creature will be unblockable during that later combat phase because the ability is on the creature prior to the declare blockers step.