The invoker set 2 piece bonus says " Each time an enemy is hit with [Punish] or [Slash] or an attack is [blocked], Thorns damage is increased by 350% for 2 seconds".
How many blocks per second and how is the number of blocks in a 2 second period determined. The way this is written the 350% increase stacks based on the number of blocks plus the number of attacks in a rolling two second period.