Now, as for counter-attacking from a block -with your blade locked with the opponents, you want to quickly disengage and attack; the key to this is to disengage in your favor.
However, a useful thing to do is consider how you blocked and how it can transition into a blow. An example is this - if I'm locked at cross with my opponent with my blade held horizontally above me, a great counter-attack is to push my opponents blade upward as I unfold my blade; this will cause their blade to go upward and to their left, not only putting their blade on the outside, but also putting the opponent off balance and putting them in a position that their strike wouldn't be able to do near as much damage.
Questions?