Your blast penalises victims in subtle and terrible ways. Instead of delivering instant Wound Points, the damage determines the severity of the curse.
Instead of normal wound points, you cause curse rating. For every 10 points of Curse Rating (dropping fractions) the victim suffers 1 point of Impairment until cured. When impairment equals the character's Constitution, he is entirely helpless, and can no longer fight the condition.
Curse Rating needs to be removed with Remove Curse, see Dispel Magic.
At the cost of a Magic point, you can do both damage and inflict a curse.
If you must always use this, and thus cannot use the blast power to do normal damage, the metablast schtick is free.
Similar to Paralysis