blur() blurs the whole picture: a pause menu over the game, being out of focus, being underwater. radius is about how far a pixel spreads, in game pixels, and amount mixes it with the sharp picture, 0 being sharp and 1 all blur.
It takes more than one read of the frame: one pass across and one down, both at half the game's size, then blown back up smoothly. So a wide blur costs about what a narrow one does, and much less than reading every neighbour at full size.
P eases amount in and out the way a pause menu would, ↑ ↓ change the radius and Space switches it off.