Oui, les deux se déclenchent "when ~ is blocked".
Par contre, je pense que l'ordre est important. Si on applique d'abord l'effet du Crypt's Sons, l'action continue "as if unblocked" et on ne peut plus jouer le Power of One (sauf si on est rebloqué).
Name: Crypt's Sons
Cardtype: Retainer
Cost: 1 pool
Unique mortal with 3 life. Requires an anarch. Whenever this anarch is blocked, he or she may burn a life from the Sons to cancel combat and continue the action as if unblocked. The Sons inflict 1R damage each round of combat during normal strike resolution.
Name: Power of One
[TR:C]
Cardtype: Action Modifier
Cost: 1 blood
Discipline: Obtenebration/Potence/Presence
Requires an anarch.
[obt] Only usable when an action is blocked. The blocking minion burns 2 blood or life (before combat, if any).
[pot] +1 bleed.
[pre] +1 bleed, or +1 bleed and +1 stealth. You cannot play another action modifier to increase this bleed amount.
Ratings coordinator et Rules Director - BCP Prince de Paris
REINS!
EDIT: Et effectivement, si tu utilises le Crypt's Sons en premier, tu ne peux pas utiliser Power of One (enfin, sauf si quelqu'un d'autre reussit le block).