i have noticed this too when the weather is overcast.If i move the camera (in external view) i see the blur effect going on/off, but the problem is that this change at very low distance over the ground and far from cloud like i think its suposed to work.
Other problem is that there are nof a fade efect, its on /off blur, so tis hard and you have the sense that there are not HDR working.
I would like to edit these effects too, but the most important things are under the hdr.fx and there are lot of code to change and i cant understand.The easy parameters are in the hdr.lua.
I have noticed too that the HDR.lua its the same for beta 4 than final, the thins that changues are in the fx one.
It would be cool than ED give us diferent configurations so we can use as we want.
PD: i didnt liked the beta hdr, too much blurry for me, but i dont like the final HRD , too much wet or aseptic.