I don't know if this will work because I haven't yet tried it, but it's an idea that came to mind over the weekend...
Take a sheet of styrofoam (those thin sheets that are used to package modular furniture would be ideal) and simply paint on the block outlines with a fine brush using thinners. The thinners should eat into the styrofoam a little, giving the effect of stone blocks. use more or less thinner as required. Depending on how rough you want the outer surface of the wall to look, you can apply more thinner as sparingly or liberally as you want to roughen the texture, or just gouge it about a little with a hobby knife, dental burr or any other tool you think might do the job. Be sure to use the thinners in a ventilated area!
Then just paint & highlight the wall with acrylics, water colours or poster paint.
Or....
Another idea that just came to mind of you want a rough wall is to break up a cork tile and use the fragments as your rocks. Glue them together with PVA, then paint & highlight.