Hidden Blade on unit with Guardian Angel, they draw?

Yes, the controller of the unit targeted by Hidden Blade will still draw 2 cards if Guardian Angel is used to save it.

According to RiftJudge FAQ #10456 and FAQ #8724, when Hidden Blade resolves, it checks if its target is still a legal target on the battlefield. Because Guardian Angel acts as a replacement effect that intercedes during the resolution of Hidden Blade (replacing the "kill" event with "Heal me, exhaust me, and recall me"), the unit remains a valid referent on the battlefield at the moment the spell begins to resolve. Since the spell successfully resolves and identifies the controller of that unit, the draw effect still occurs.

Sequence

1. Hidden Blade is played, targeting a unit on the battlefield.
2. Hidden Blade begins to resolve.
3. Guardian Angel's replacement effect triggers, replacing the "kill" event with "Heal me, exhaust me, and recall me."
4. Because the unit was a legal target on the battlefield when Hidden Blade began resolving, the spell successfully completes its resolution.
5. The controller of the unit draws 2 cards.