If you want to be able to customize the blurred background effect in the folder-view in iOS 7, you have to come to the dark side a.k.a jailbreak your iPhone. And it's interesting how, with simple Cydia tweaks, you can totally change the way the folder UI looks on your iPhone/iPad.
By default, the folder UI puts a glass-like background beneath the icons in the folder. This creates a blurred-background effect. Whether you like it or not is a personal opinion but if you would like to experiment with various options, take a look at these Cydia tweaks.
Remove Background Completely
Tweak: ClearFolders | Price: Free | Repo: ModMyi
I'll admit one thing. Having no background for folders seems more in line with the minimalist attitude of iOS 7 than anything. Perhaps that's what makes a tweak like ClearFolders instantly likable.
With ClearFolders installed, folder UI will have no background of its own. So the blur effect is completely removed leaving you with the original homescreen background. Try it out and see how minimalist your folder-view gets and how it matches well with iOS 7's general interface aesthetic.
Blur the Whole Background
Tweak: BlurFolders | Price: Free | Repo: BigBoss
If you feel that partial blur is what looks odd, may be a full-background-blur effect would look okay, then BlurFolders is a tweak you should try. BlurFolders extends the blurred background of the folder UI to fill the screen completely. As shown in the image, the blur effect is applied to the homescreen so you have a smooth, totally blurred background for the folder view.
Since the blur is now uniform and evenly distributed, the UI looks quite good. In my opinion, better than the default view.
Optimal Blur Level
Tweak: ReplaceBlurs | Price: Free | Repo: BigBoss
What if the blur effect was replaced by a simple white-screen-effect, an overlay of translucent white over the background? That is precisely what ReplaceBlurs does. Instead of the blur effect, all you have as the folder background is just a translucent white background. The homescreen background filters through it. The effect is neat.