When attacking with 5-cost Rek'Sai (which allows playing a card from the top of deck), can you play a spell like Hidden Blade and play it hidden?

Ruling: You can play any card from the top of your deck with Rek'Sai's ability, but you cannot hide a card played this way. Hiding is only permitted with cards in your hand.

Nuances:

• The restriction on hiding applies even though it may not be explicitly stated on the card itself