If you draw and play Hidden Blade via Reksa's legend ability to reveal the top 2 from deck, can you still play it as a hidden to a battlefield you control?

No, you cannot play Hidden Blade as a "Hidden" card if you are playing it via the effect of the Rek'Sai (Void Burrower) legend ability.

Here is the breakdown of why this interaction does not work:

1. Hiding is a distinct Discretionary Action: According to Rule 421 and the keyword rules for Hidden (Rule 811), "Hiding" is a specific Discretionary Action. It is not the same thing as "Playing" a card.
2. Prerequisites for Hiding: The Hidden keyword (Rule 811.1.b) specifically allows you to hide a card only while that card is in your hand or your Champion Zone.
3. Rek'Sai's Ability: The Void Burrower legend ability instructs you to reveal cards from your Main Deck and then play one of them. Because the card is being played from your Main Deck—not your hand or Champion Zone—you do not meet the prerequisites to perform the "Hide" action required by the Hidden keyword.

Because Rek'Sai's ability specifically instructs you to play the card, you must play Hidden Blade as a spell (paying its costs as normal) rather than attempting to hide it.