Last night the Hearthstone subreddit discovered that the Korean Refund Page states that Blackrock Mountain preorders can be refunded until the adventure opens on April 3rd (6am KST) for Korea, which would mean Blackrock Mountain is launching April 2nd for US/EU players.

The text was later removed but screencaps remain.

Picture of translation here:

Do you think the date is real or a placeholder? This would mean Blackrock Mountain is opening on a Thursday. Usually Blizzard updates are on Tuesdays, but we saw all Curse of Naxxramas wings open on Wednesday.