When I have one unit at a battlefield and the enemy is playing Hidden Blade and targets this unit, what happens when I play Tideturner from Hidden from the other battlefield and switch with my unit that was targeted by Hidden Blade before?

Whether your unit is successfully saved from Hidden Blade in this scenario depends on how that Hidden Blade was played.

According to Core Rule 811.1.d.2 and RiftJudge FAQ #210, #2461, and #10852, when a spell is played from "Hidden," it is restricted to targeting a unit at the specific battlefield where it was hidden.

The Interaction Outcome

1. Hidden Blade is played/activated: It targets your unit at Battlefield A.
2. Tideturner is played from Hidden: You play Tideturner at Battlefield B in response. Its "When you play me..." ability goes on the chain.
3. Tideturner ability resolves: You choose your unit at Battlefield A and swap it with Tideturner at Battlefield B.
4. Hidden Blade resolves:
   • If Hidden Blade was played from the opponent's hand: It does not mistarget. According to FAQ #7179, a standard Hidden Blade played from the hand only requires the target to be a unit at any battlefield. Because the targeting relationship is established with the unit itself, moving that unit to Battlefield B does not break the targeting. Your unit will still be killed at its new location.
   • If Hidden Blade was played from "Hidden": It mistargets. Because Hidden Blade was played from hidden, it possesses the restrictive targeting requirement that the unit must be at the battlefield where the spell was hidden (Battlefield A). When your unit moves to Battlefield B, it no longer meets this requirement. When Hidden Blade resolves, it "mistargets" and has no effect: your unit will survive, and no cards will be drawn.

Summary

• Targeting Restriction: Rule 811.1.d.2 dictates that hidden spells must target units at the battlefield where they were hidden.
• Mistargeting: Rule 359.3.e.9 defines that if a target becomes invalid (e.g., it moves to a location that no longer satisfies the spell's restriction), the spell mistargets.
• Tideturner Exception: Per FAQ #7907 and #6517, Tideturner is a unique case that is allowed to target units outside of its hidden location because its effect would otherwise be impossible to fulfill. This makes it an effective tool for "dodging" effects restricted to a specific battlefield.