Blood Brother Ambush

Name: Blood Brother Ambush
[Sabbat:R, SW:R/PB]
Cardtype: Action Modifier
Clan: Brujah antitribu
Cost: 2 blood
Only usable when an action is blocked.
Combat does not occur. Put this card into play. While in play, this card represents an ally with 3 life and 2 strength who can strike for 2R damage; this ally enters combat with the blocking minion. Blood Brother Ambush may play cards requiring basic Potence [pot] as a vampire with a capacity of 3. If a card would give them blood, give them life instead. Burn this card at the end of combat or if the combat is canceled.
Artist: Ted Naifeh

I miss some inportant information on this card: What happens to the announced action, e. g. does it end? And when exactly? What happens to the blocker, e. g. does he become locked?

Card text of superior "Mirror Walk" for comparison:

Name: Mirror Walk
[Third:C, KoT:C, SP:PwN6]
Cardtype: Action Modifier
Discipline: Thaumaturgy
Do not replace until your discard phase.
[tha] +1 stealth.
[THA] As [tha] above, and if this action is blocked, lock the blocking minion and end the action before block resolution.
Artist: Brian LeBlanc

Thanks in advance!

The action was blocked, move to the part of the action where combat occurs. Then it doesn't occur and the BBA combat happens.

The acting minion is locked, as they were for taking the action, and are the acting minion for the action still, until conclusion of the BBA stuff.
Blocker is locked (unless they have atonement or GV @ ,etc. ) and fight the BBA created thing.

MW specifically ends the action. Its replacement effect specifically supplants block resolution, ends the action, taps (locks) blocker.

BBA doesn't, its replacement effect only impacts the combat, supplanting normal combat for its combat during the block successful part of the action.