You control the intensity of the blur by setting the size of the blur radius. Larger radii give more blurring. Smaller radii give less blurring. For the purpose of editing cave maps, blurs in the range of 0.5 are usually best.
As an example of how the blurring works, the image to the right was was converted to black and white. This causes the subtle variation of color around the edges of the lines to be lost, (although it does quickly remove a lot of extraneous detail). As you can see, the image looks overly sharp and stark.