Interesting, thanks for the link. Seems like they use that same “magic” wallpaper drawing I mentioned earlier and implement the blur by setting a blur property on the activity’s window, therefore letting the system compositor do the blurring. Had no idea that was possible.

Still has some limits, though, like inability to blur specific parts of the image (e.g. under a box) and apparently the Android skin that ships with some devices disables that window blurring feature, in which case it won’t work. Better than nothing but still restricts the scope of what’s possible quite a lot.