An assessment action usually takes quite a while, that's why you usually don't do it during a conflict, but long before, to prepare for a conflict. In the case of the counterspell, the assessment is part of the action, not actually a free action. When you do a counterspell, you always do the assessment action at the same time.
However, I would only allow it like that if the wizard in question opens his sight. That way, he has a clear look at the spell and can attempt to counter it, much like Harry did with the barbed wire spell in GP. Without the sight, the assessment action would take longer.