As I understand it, blocking/countering on reaction is unusual even in high level play. What people do is make educated guesses on which move the opponent will perform after the one that is currently being displayed on screen, either based on which combo would deal the most damage, or which combo the opponent has used in a similar situation before. If you know that the enemy knows that a certain combo would completely decimate you, you can predict his move ahead of time. Of course, if the enemy knows that you know, he might switch from the combo that would defeat you, to one that wouldn't defeat you to avoid your counterattack.