Area attacks are governed by their own rules:
Origin Square: You choose a square within an area attack’s range as the attack’s origin square, which is where you center or start the area of effect. You need line of effect from a square in your space to the origin square. For a target to be affected by an area attack, there needs to be line of effect from the origin square to the target. You don’t have to be able to see the origin square or the target, and concealment between the origin square and the target doesn’t apply.
Therefore we need something called "line of effect" but any kind of concealment doesn't matter.
Line of effect is:
A clear line from one point to another point in an encounter that doesn’t pass through or touch blocking terrain. Unless noted otherwise, there must be line of effect between the origin square of an effect and its intended target for that target to be affected.
From a trivial case, blocking terrain blocks line of effect:
A type of terrain that blocks squares, often by filling them. Examples: Walls, doors, and large pillars. Blocking terrain provides cover, interferes with movement around it, and blocks line of effect. It also blocks line of sight, unless it’s transparent.
Therefore, since blocking terrain is a separate feature to cover (though cover is a way of determining "not quite blocking" terrain) enemies who merely grant cover do not count as blocking terrain. Therefore they do not block line of effect.