Phred, the flat silhouettes I sell are full sized, with removable stakes to mount the birds on the ground.
$24 for a dozen includes shipping, so it's $2 a bird vs $1.17 ea for the walmart decoys. The 12 birds stack up to 2" high - lightweight, waterproof, weatherproof.
Here's two places you can get 'em -
http://www.ebay.com/itm/320789587475?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1555.l2649
http://www.hangtimeoutdoorproducts.com/ <~~~~ front page picture is our own Joe with his spread and a couple of dead birds
Heck, I remember when I first started researching these things one guy said he spray painted empty 2liter bottles black and threw 'em on the ground and got results. Mixing in more black with your existing decoys can't hurt. Only two problems I see with your plan is 1) that the size might be more like a big starling/blackbird instead of a crow and I don't know how they react (don't really know or have any experience with that) and 2) painting (to me) is just a hassle to be avoided. I'm just lazy.
But will your plan work? I'd say maybe, probably....most likely it can't hurt.
What a pain in the balls that crow season is closed until June 1st. I really hate that.