When Hidden Blade is played from hidden, can it target units at any battlefield or only the battlefield where it was hidden?

Ruling: When Hidden Blade is played from hand, it can target a unit at any battlefield. When played from hidden, it can only target units at the battlefield where it was hidden.

Nuances:

• Hidden cards that require a target must pick from the battlefield they were hidden at, unless the card explicitly restricts targeting in a way that makes it impossible to target something on the current battlefield (like requiring a target at a different location)
• Saying "any battlefield" would be equivalent to "a battlefield" and doesn't prevent targeting the current battlefield, so it wouldn't allow targeting other battlefields when played from hidden
• If Hidden Blade is played from hand targeting a unit at one battlefield, and that unit moves to a different battlefield before resolution, the target is still valid because it's still at a battlefield