All characters share the same blast attack commands.
The weaker blast attack can be performed by moving the dpad a half circle from back, down, to forward+ki blast button. This will fire the attack so long as you have any ki left in your characters charge meter.
The stronger blast attack can be performed by pressing down, back, forward+ki blast button. This requires more energy to fire, but is the most powerful.
For reference, i'll label these attack Blast 1 and Blast 2. As said, all characters share these commands, though the moves themselves differ from player to player.
If an enemy fires a blast 1 or blast 2 attack, you are given a split second chance to counter it with a blast of your own.
If blast 1 is fired (half circle+ki blast), counter with blast 2 (down, back, forward+ki blast). Likewise, if blast 2 is fired, counter with blast 1.
If countered successfully, your character will engage a blast fight battle. The player that taps the ki button the most within the short animation sequence will win, and the loser will be hit with the other player's blast attack as if it were blast 2.