Yesterday I noticed that when a blaster fired at creeper on one side of a crazonium wall (in this case capping an emitter) it also destroyed creeper along the far side of that wall. (a strip one square wide by several long along the wall.)
This occurred on this (http://knucklecracker.com/creeperworld/mapcomments.php?id=617&page=) custom map - while capping the farthest left emitter along the top edge. I haven't had a chance to find out if it happens on other maps or with regular walls.
It is a known phenomenon of blasters (and mortars) that their damage area can extend through barriers. Virgil acknowledged that.