how to create an area effect of changed blocks

There's a procedure template called 'check for block in 6*6 cube' that can be pretty easily modified to do just this. Essentially, set the radius to whatever you want, (it has a 6*6*6 cube by default), making sure that the repeat blocks are double whatever you set the variables to. Then, for each block, instead of changing the 'found' variable, just execute whatever procedure you want to take place at that location.

There's all sorts of stuff you can do with this, from simply making a cube, to adding randomization to spread patches of stuff. The size and shape can also be changed pretty easily. If you only want something on the surface level, you can also swap out the Y offsets with the World Surface Level to make the thing run a bit more smoothly for larger stuff.