Blurry Reflection
Blurry Reflections are cause by setting the Sharpness settings on the Material Wizard to a low value.
A high value (1.00) set the reflection to be perfect - e.g. for mirrors. A value such as 0.90 lets in a little blur. And lower values let in more and more blur.
More blur requires more passes
For this scene, I have intense light coming in through the window. The direct sun goes to the left, and the center reflection is reflection from the sky. I have set sharpness to 0.15 which creates a very blurry - but sometimes realistic - reflection.
5 passes
After just 5 passes, you can see how the renderer created the blurriness. It has sampled the window reflection as 5 separate pieces - one for each pass. With high blur (low Sharpness), and a low number of passes the looks very unrealistic.
40 passes
Even after 40 passes, the effect is not realistic:
1,000 passes
However, after 1,000 passes the blur has converged much better and is producing the result desired.
Conclusion: Special lighting effects are going to require lots of passes to have time to converge properly.
Another Example
Here is a sample where the road is marked to have blurry reflection.
The final image, here after 500 passes, has a very blurry effect.
Here is how this is done:
After the first pass, these is just one single sharp reflection on the road:
However, in the second pass a second sharp reflection is generated - creating a choppy effect.
But after this is repeated, over and over, the final blurry reflection is created:
