The blur happens in the compositor. It doesn't happen in the secure windows.
>A secure window should draw 100% as requested or not at all.
Take for example "night mode" which adds an orange tint to everything. If secure windows don't get such an orange tint they will look out of place. Being able to do post processing effects on secure windows is desirable, so as I said there is a trade off here in figuring out what should be allowed.