Can hidden cards be played at any eligible timing, or only when a showdown is happening at that specific battlefield?

Ruling: Hidden cards can be played at any eligible timing when you have priority, as long as the card has not been placed facedown during the current turn. They are not restricted to only being played when a showdown is happening at their battlefield.

Sequence:

• Opponent plays a card (like Blitzcrank) that creates a trigger on the chain
• You gain priority to respond
• You can play the hidden card as a reaction to that trigger, regardless of which battlefield the opponent's card was played to
• The hidden card gains the extra text "here" added to its effect, meaning it affects units at its own battlefield
• The hidden card resolves before the opponent's trigger

Nuances:

• All hidden cards have reaction, so they can be played during an opponent's turn
• Hidden cards gain "here" text when played, restricting their effect to their own battlefield location
• You cannot play a hidden card if it was placed facedown during the current turn