A well-timed block does not differentiate what kind of attack is incoming. You can even parry special attacks (although Kabam nerfed that by making specials stun-immune). Heavy attacks are not stun-immune.
When combined with the SYG mastery, you can have a well-timed block resist a heavy attack and generate a parry and then stun.
It is extremely rare because you have to be fighting a champ with those masteries (only in arena or AW), most players don't run the SYG mastery (or only have 1 point in it so don't have a high percentage chance to resist), the timing of a parry against a heavy is very difficult to achieve, and the AI doesn't actively try to parry.
In theory, you could even parry a heavy attack and not generate a stun (if opponent has ability accuracy reduction, like BW or Xbones).
Comments
When combined with the SYG mastery, you can have a well-timed block resist a heavy attack and generate a parry and then stun.
It is extremely rare because you have to be fighting a champ with those masteries (only in arena or AW), most players don't run the SYG mastery (or only have 1 point in it so don't have a high percentage chance to resist), the timing of a parry against a heavy is very difficult to achieve, and the AI doesn't actively try to parry.
In theory, you could even parry a heavy attack and not generate a stun (if opponent has ability accuracy reduction, like BW or Xbones).