If you put them in a tupperware box with some gravel and shake them it does the same thing, and it makes small surface dents etc. that 'age' the stone blocks. The more vigorous you shake it the more dented they get. You can also roll a handful together between your hands to round off the edges.
Thats a great tip Mark. I had also considered rubble filled walls, like yours, but it would be too thick for what I have in mind.
Unfortunately I had already glued together most of one ruin before I read your post, so have continued with my original plan, 'ageing' the blocks with crunched up baking foil.
First ruin with untextured blocks:
Second ruin with textured blocks:
Ground texture and construction of the scaffolding to do next.