I really like that! Although I could see how the sharp edges of the tiles might be interpreted as non-background pixels. So I still wonder if blurring the background might be worth trying.
My usual approach so far is to make the background significantly darker than the foreground. Examples:
(http://neolemmix.com/old/levelimg/lpv/0407.png)
Unruly 7 "Lemming Workout Routine"
(http://neolemmix.com/old/levelimg/lpv/0502.png)
Outrageous 2 "Working Bee"
(http://neolemmix.com/old/levelimg/lpv/0314.png)
Edgy 14 "Unstable Formation"
(http://neolemmix.com/old/levelimg/lpv/0405.png)
Unruly 5 "Ancient Technology"
It's worth noting that from the next update onwards, NeoLemmix also features a button you can press (configurable as either "press and hold" or "press to toggle") that replaces the normal display with one that has no background, and simply renders terrain as light gray and steel as dark gray. See here (http://www.lemmingsforums.net/index.php?topic=2894.msg60239#msg60239) for images (check both the linked post and the one after it).
I also intend to add (but haven't done so yet) a "Disable Background Images" user-side option.