Tenebrous form vs Second Tradition

Charles_Bronson wrote:

It's strange because the wording on the card does not say you have the option to back out. Only say that you MUST TO.

But attempts to block are optional, but you usually can't back out of a second tradition domain attempt to block. So why can you now back out?

"must do X TO do Y" is a cost for an effect, which leaves the player the choice to pay or not.
It's different from "vampires attempting to block must burn 1 blood" (or "burn 1 blood" for that matter, "must" being redundant) for instance which wouldn't be a cost but an effect.

Also, the condition to attempt to block is not met anymore as soon as Tenebrous Form is played, and as long as the blocking minion has not paid.
That's why the blocking minion isn't attempting to block anymore if they don't pay that blood "to attempt to block" (again).