These are a good idea - but your algorithm is wrong, it's not a true blur. What it's really doing (and it's obvious from the shader code that it does this) is additively blending a set of offset images. This looks like a blur for low radius values, but doesn't work for high blur values. If the sampling points are more than one pixel apart, then a kind of stepped shadow becomes visible.
<img src="http://www.scirra.com/images/blurs.png">
This is why I haven't added variable blur radius to the existing blur horizontal and blur vertical: if the blur value goes too high, you get shadowing, and if it goes too low, you're sampling the same pixel more than once, which is inefficient. A true blur has to take in to account every single pixel over a range.
They're still useful though, and something like Motion Blur would be cool to adapt in a way like Blur Horizontal works