Blockstun is the kind of stun incurred by the blocker when they block a strike.
Unlike pushback (“hitstun”) and other kinds of stuns, you can relieve yourself from a block in some games after the mandatory blockstun time (by letting go of the block position on the directional input); this allows you to attempt other things, if the opponent leaves a sufficient opening. Alternatively, you can cancel all or some of your blockstun time with a guard cancel technique.
Some games will “force” you to continue blocking (it’s not really a bad thing per se) until you are truly ready to do something (see continuous block), which is a different blockstun behavior, really.