Can Hidden Blade be used as a reaction to destroy an opponent's unit the same turn it was played hidden, if that opponent's unit is targeting your unit?

Ruling: No, you cannot use Hidden Blade as a reaction the same turn you play it hidden, even though it becomes a reaction card when played hidden.

Nuances:

• Hidden Blade does become a reaction type card when played hidden
• However, you cannot play a card from hidden the same turn you played it hidden
• This timing restriction prevents the interaction described in the question