Can hidden cards on one battlefield interact with or affect the opposite battlefield, specifically can Hidden Blade hidden at one battlefield be used to aid an attack on the opposite battlefield?

Ruling: Hidden cards gain a targeting restriction when played - units must be played to the battlefield they're hidden at and spells must target things at the battlefield they're hidden at. Hidden Blade must target a unit at the battlefield it's hidden at, though it can be revealed and played any time the owner could play a normal reaction.

Sequence:

• If you have Hidden Blade hidden at battlefield A and attack battlefield B
• You can still reveal and play Hidden Blade during the attack at battlefield B
• However, Hidden Blade must target something at battlefield A (where it's hidden)

Nuances:

• A few cards create global effects from hidden that bypass this restriction (Zhonya's Hourglass, Tideturner, and Stand United)
• 99% of cases follow the standard targeting restriction rule