Can a hidden card (like Hidden Blade) be revealed at reaction speed during an opponent's turn, or must you wait until your own turn to reveal it?

Ruling: You can reveal and play a hidden card during your opponent's turn as a reaction.

Sequence:

• Turn 1: Play the card hidden at a battlefield
• Turn 2 (opponent's turn): You can reveal and play it as a reaction (e.g., when they attack that battlefield)

Nuances:

• Hidden cards are not considered "played" when you initially hide them at a battlefield