Wild Swing
If you cannot Perceive a target, at the GM's discretion you may make a Wild Swing. This is most successful with Melee Attacks, but can occasionally be used on Ranged and Area attacks as well. This involves guessing at a target's location, usually their last known location (i.e. where they were before they become Hidden or where their last attack originated from), or choosing a random location to attack. When making a Wild Swing, you then choose a location (or direction) to target and make an attack (accounting for Cover and Concealment as normal). If a creature is in the attacked location, the attack will then hit or miss as normal. If no creature is in the attacked location, then the attack automatically misses.
When using a power that targets an Area, you may attempt to target a location that you cannot Perceive (due to Blindness or Darkness and other environmental effects). In this case, you determine the distance and direction of the area's origin (or choose at random). At the GM's discretion, you may be required to make a Wits check in order to place the area in the desired location. If you fail the check, the GM may shift the Area of Effect at his discretion.