I can confirm that this works for ARBG images and I am using alpha in those images. If you need the blur to spread even more, you can resample the image down, blur, then resample up again at high quality. You can balance the look and quality of the down/upsample with the performance gained by blurring fewer pixels, so combining these techniques has been really interesting for CPU-based blur and saved me from the complexity of implementing OpenGL (for now).