If Hidden Blade's target leaves the battlefield before the spell resolves, does the controller of the units still draw 2 cards?

Ruling: No, nobody draws 2 cards. If the targeted card leaves the battlefield, it is no longer a legal target.

Nuances:

• When a target leaves the battlefield, there is no longer a "controller" because there is no target
• The only exception is if a card moves zones as a cost or effect - only then can the spell "look back" at its characteristics before it changed zones, but this is a very restricted form of last known information