If an opponent plays Hidden Blade on my unit and I retreat it into my hand in response, do I still draw 2 cards?

Ruling: No, you do not draw 2 cards. If the target unit is no longer a legal target on the battlefield when Hidden Blade resolves, the effect cannot find "its controller" and therefore no cards are drawn.

Nuances:

• If a unit is moved to your base (still on the board) after being targeted by Hidden Blade, you would still draw 2 cards because the unit is still a legal target and its controller can be identified
• A spell or ability that moves something to a different zone as a cost or effect can "look back" at its characteristics before it changes zones, which is why Retreat itself can still channel a rune even though the unit goes to hand
• The key difference is that Retreat says "owner" (not "controller") and Retreat itself is doing the move, allowing it to look back in time
• If a unit leaves the board through other means (like a replacement effect), the information may not be accessible