If Hidden Blade is hidden at a battlefield, can you react with Hidden Blade to kill another unit at a different battlefield?

Ruling: No. Spells and play effects of units played from hidden can only target game objects at the battlefield where they were played from unless the targeting specifies otherwise.

Nuances:

• This restriction only applies to targeted effects; untargeted effects (like the second part of Stand United) don't have this restriction
• Play effects that explicitly specify another battlefield (like Tideturner) are exempt from this restriction
• While saying hidden effects add the word "here" is a common shorthand, it's not technically accurate because it wouldn't correctly describe untargeted effects or cards that specify other battlefields