It doesn't come out the same. You either aren't looking at it magnified enough. Or, for tiny displayed images, you'll need to increase the blur window to notice a difference. This code works:
rgbImage = imread('peppers.png');
rgbImage = im2double(rgbImage);
windowSize = 15;
avg3 = ones(windowSize) / windowSize^2;
subplot(2,1,1);
imshow(rgbImage)
blurredImage = imfilter(rgbImage, avg3, 'conv');
subplot(2,1,2);
imshow(blurredImage)

