If I use Flash on my unit after it is being targeted by a Hidden Blade, do I still draw 2?

No, you do not draw 2 cards.

According to the RiftJudge FAQ (ID 2778), if you use Flash to move a unit targeted by Hidden Blade back to base in response, the target becomes illegal because it is no longer at a battlefield. When Hidden Blade resolves, it cannot find the target at the battlefield, which makes it impossible to determine "its controller." Consequently, the draw effect does not trigger.

Sequence of Events:

1. Player A plays Hidden Blade targeting a unit at a battlefield.
2. Player B (you) responds with Flash, returning that unit to base.
3. The chain resolves in reverse (LIFO). Flash resolves first, moving the unit.
4. Hidden Blade attempts to resolve. It checks for its target at the battlefield.
5. Because the unit is no longer at the battlefield, the target is illegal. The instruction to "Kill a unit at a battlefield" fails, and the instruction to "Its controller draws 2" cannot be executed because the spell cannot identify the controller of a unit that is no longer at the required location.

This is consistent with Rule 359.3.e.12, which states that if a spell checks information about a target that is no longer legal or whose location has changed such that the information is unavailable, that check returns "null" and all calculations based on it are ignored.