What happens if I target a unit at a BF with Hidden Blade. In response, my opponent plays a Tideturner from Hidden from the other BF and swaps places with Tide and the unit I targeted with Hidden Blade. Does it still die considering it's not on the same BF now?

It depends on how the Hidden Blade was played.

• If Hidden Blade was played from hand: The unit still dies. Hidden Blade from hand only requires "a unit at a battlefield" — not a specific one. The targeting relationship follows the unit itself, so after Tideturner swaps it to the other battlefield, Hidden Blade tracks it there and kills it. Its controller draws 2. (FAQ #7179)

• If Hidden Blade was played from Hidden (facedown): The unit survives and no cards are drawn. Hidden spells have an extra targeting restriction — the target must be at the same battlefield where the Hidden card was hidden (Rule 811.1.d.2). Once Tideturner moves the unit off that battlefield, it no longer meets that restriction. Hidden Blade mistargets on resolution and does nothing. (FAQ #210, #2461)

Tideturner is a special case — its ability explicitly allows it to target units outside its own battlefield when played from Hidden (per card text and FAQ #7907), so the swap itself works fine either way. The question is whether the swap saves the unit, and that answer depends entirely on how Hidden Blade was cast.