What does the blobby icon in Hexcells Infinite do?
It toggles an outline for the blue counter zones. The "there are this many blue cells within 2 cells of this one" zone hint, that's introduced in 4-1.
Screenshots below, first showing zone without the highlight, second showing zone with the normal highlight, and third showing it with the border.
Took me a bit to tinker with the game and find out what it was doing. Wish I'd noticed that earlier, would've saved a lot of squinting!