If Sett with a buff gets targeted with Hidden Blade, can he use Sett legend to save him, recall him but still draw 2?

Yes, you can save the unit and still draw two cards.

According to RiftJudge FAQ #2372 and FAQ #11031, when you use Hidden Blade on a buffed unit and use Sett's (The Boss) legend ability to save it, the draw effect still occurs.

Here is the breakdown of why this works:

1. Replacement Effect: Sett’s ability is a replacement effect that intercedes during the resolution of the "kill" instruction.
2. Tracking the Controller: Because the replacement effect only modifies what happens to the unit (recalling it to base instead of sending it to the trash), the unit remains a valid referent for Hidden Blade.
3. Successful Resolution: Since the unit was a legal target when Hidden Blade began resolving and the controller can still be identified, the draw instruction of the spell successfully executes.

Important Note: The unit must be at a battlefield when Hidden Blade begins resolving for the spell to target it legally. As long as you use Sett's ability during the resolution process to replace the "kill" with a "recall," the controller is still identifiable, and you get your draw.

This interpretation is based on the interaction between replacement effects and spell resolution as described in the FAQ database. No official rule change has altered this interaction.