Blinding duration critical?

From rank 8 onwards there's a training requirement that states "Succeed in blinding the opponent for a long time." Is there some sort of critical effect where sand burst can blind for longer than normal? Or does this just mean you need to use an clay cylinder? Also wondering if blinding duration increases at all with higher skill ranks, not just alchemy bonus from Alch mastery and clay alchemy.

This requirement for me can also be fulfilled with a normal cylinder, but the requirement gets training occasionally from time to time with consecutive sand burst use. The requirement can be fulfilled with or without critical effect.

From what I noticed with my friend, it seems to happen often without a critical. Maybe because he has rank 5 sand burst. I'm not sure what it's determined by other than the training count appearing in the system chat.