Does Hidden Blade draw if opponent's Flight or Flight its unit?

No, you do not draw 2 cards if your opponent uses Fight or Flight to move their unit to their base in response to Hidden Blade.

According to the established rulings for this interaction:

• Target Legality: When Hidden Blade resolves, it checks for a valid target. Because Fight or Flight moves the unit to the base before Hidden Blade resolves, the unit is no longer "at a battlefield." This makes the target illegal.
• Resolution: When a spell resolves with an illegal target, the effect fails to apply. Because the target is illegal, the game cannot determine "its controller" to resolve the draw effect. Consequently, the unit is not killed, and no cards are drawn.

This is a standard defensive play in Riftbound: moving a unit out of the battlefield in response to a targeted effect that requires a unit at a battlefield makes that target illegal, causing the spell to fizzle.