Not sure I understand it's action right. When use it who discards cards from the deck. Player who uses the action or his opponent?

Blackmoor Estate reads

Action: Exhaust and pay X to discard the top X cards of a player's deck.

Since the ORD defines targetting as "The word target is used to indicate that an effect is directing a player to choose 1 or more cards for an effect to resolve on."  I'm going to assume that they chose not to use target, and that you are selecting (not targetting) a player for the effect.  However, I'm in agreement that it's written strangely.  It should likely say "Choose an opponent", but since CoC is pretty much 'as intended' then my assumption is that it's written so that you can choose any opponent or yourself for the effect.

The player triggering the Action gets to decide, and yes, she can also choose herself.

This isn't just some weirdo rules loophole.  You often WANT to choose yourself.  Yog has a lot of great recursion cards that can make good use out of your discard pile, so why wouldn't you want to load some good stuff in there?

So, Blackmoor Estate can be used in a mill deck, or in a deck that has some good disard recursion and wants to set itself up.