Then what is your problem? Don't you wish that the sky is blurred? If not, you can solve it rather technically by using something like clip_far - 1 as threshold and write 0 instead into the depthmap, or you create a buffer, in which you write 0 and 1, which parts of the scene should be combined with the blurred image (where the sky would be black then) - you take this mask as lerp factor between unblurred and blurred image in the composite pass.