If you receive a critical strike, subsequent critical strikes will NOT stack on top of your existing Blessed Recovery. Instead, it will refresh the ability with the value of your last critical strike. For example, if you take a 3000 damage critical hit,you will receive the Blessed Recovery effect for 750 damage (assuming max rank). If you take a 1000 damage critical strike directly afterwards, it will overwrite your current Blessed Recovery with a 250 damage restore.
This talent is mainly useful in PvP against rogues, warriors and hunters. However, due to the lack of a stacking effect, its effect is somewhat lessened.
This talent is also improved to some extent with Blessed Resilience. It guarantees that you heal for the critical strike damage and not have another one overwrite it since you cannot be critted on for 6 seconds.