A stun blast does no direct damage. Instead, targets lose three shots if the blast would normally have inflicted any damage. A stun blast that inflicts Wound Points equal to the target's soak attribute also suffers some Setback; the default result is that you lose shots equal to the full outcome.
At the cost of a Magic point, you can do both damage and stun.
If you must always use this metablast, and thus cannot do normal damage, the metablast schtick is free.