The basic idea is to overlay [the brushed area to be blured] with the same area in actual image by using "source-in" composition, then draw this blured area back on top the original image.
But one tiny problem with the original example is that it each time overlays the selected area with the original image instead of the updated one from last blurring, so the result of each brush does not add up. I modified the fiddle by adding another hidden canvas to keep account of the updates.
