Just to add to what Oshyan is saying, procedural based patterns don't have pixels, but rely on shading an object based on it's position in space. When you blur a raster based image, (very simply) the values of surrounding pixels are summed up and averaged. The output of that average is the new value of the pixel being blurred. Basically it works because there is a finite amount of pixels to calculate. A 24 x 24 image has 576 pixels, so there would be 576 calculations to do.
But procedural shading works in 3D space, and relies on shading points in 3D space. Because there is (theoretically) an infinite number of points, accounting for procedural blurring is a much harder thing to calculate.
There are methods to "blur" procedurals though. If you have ever written shaders for Renderman, for example, there is a handy function called "filterwidth", that does something similar to a 3D "blur". But it's not a true 3D "blur" as it needs pixels, and doesn't work in 3D space. It takes the float value, and returns the approximate amount of the average of pixel values surrounding the float value.
