Dominion and attempt to block unlock cards

beslin wrote:

Dominion

Master.

Put this card on a Black Hand vampire. Vampires must burn 1 blood to attempt to block this Black Hand vampire. A vampire can have only one Dominion.

If vampire with Dominion anounce any action at +1 stealt action,vampire B play any card with text: unlock and attempt to block,but without intercept

Yes, be there a Dominion or not (Dominion only forces B to have blood to burn in order to be able to declare a block attempt).

beslin wrote:

,so block immediately fail.

The block doesn't immediately fail: the blocking minion can play intercept.

beslin wrote:

Does vampire B still need to burn blood due to play card with text: unlock and attempt to block?

Yes. B MUST burn 1 blood to attempt to block. So if B does not have blood, he cannot attempt to block in the first place (and cannot play the card).
If B has blood, he must burn 1 blood when he attempts to block (that is when he plays the card).