If my opponent uses Falling Star, can I use Hidden Blade on my own unit?

Yes, you can use Hidden Blade on your own unit in response to Falling Star, provided the Hidden Blade is already Hidden at that battlefield.

Here is the breakdown of how this works:

1. Timing: While Hidden Blade is normally an [Action] (which cannot be played in response to other spells), the Hidden keyword (Rule 737.6) grants the card [Reaction] while it is facedown. This allows you to reveal and play it at any time a reaction is allowed, including in response to an opponent's spell like Falling Star.
2. Targeting: Hidden Blade targets "a unit at a battlefield." It does not specify an "enemy" unit, so you are free to target your own unit.
3. Resolution: If you kill your own unit with Hidden Blade before Falling Star resolves, the following happens:
   • You will draw 2 cards (as you are the controller of the killed unit).
   • When Falling Star attempts to resolve, if the unit it was targeting is now in the trash, that specific instance of damage will fail to deal damage to that unit because the target is no longer on the board.

Important Note: You cannot play Hidden Blade from your hand to react to Falling Star, as it is an [Action] in your hand. It must already be hidden on the battlefield where your unit is located (Rule 737.1.d.2).