If Hidden Blade targets a unit at a battlefield, but the opponent uses Flash to return that unit to base in response, does Hidden Blade still draw 2 cards?

Ruling: No, Hidden Blade does not draw any cards. When the target becomes illegal (unit returned to base), all information about the target returns null, including owner information.

Sequence:

• Player B plays Hidden Blade targeting Player A's unit at a battlefield
• Player A responds with Flash, returning the unit to base
• When Hidden Blade resolves, the target is no longer at the battlefield (illegal target)
• The game returns null for all information about the illegal target
• Null owner draws 2 cards (which means no cards are drawn for the actual player)

Nuances:

• The key restriction is "unit at a battlefield" - even though the unit still exists in the game, it's no longer at the required location
• If the unit moves to a different battlefield (e.g., via Ride the Wind) and Hidden Blade was played hidden targeting the original battlefield, the target also becomes illegal
• If Hidden Blade was played from hand (not hidden), targeting a unit at a different battlefield would be valid