There’s been an image floating around on the Illustrator subreddit (a still from this looping animation, it turns out) with a lot of stabs at duplicating it in Illustrator. Today, a pretty good recreation got posted. I followed their instructions and got a pretty decent version, then decided to see if I could squish it down from four overlapping blends of two squares to one blend with some complicated Appearance stacks, so it would be a lot easier to re-use.
I think I did a decent job; here’s my source file. All basic Illustrator for once.
And this was the results of trying those complex Appearance stacks on more arbitrary shapes. The one on the left has “knockout group transparency” turned on, which is the default when blending shapes together. Looks like some kind of sex toy, really.