This is interesting and really counterintuitive for me after seeing the results.

I would assume (and still can't shake the feeling) that a blind person would clearly know that a cube/square has corners and a sphere/circle does not. It's hard to understand how that isn't obvious when seeing for the first time. Even if it wasn't immediately understood, one should be able to trace, with fingers or eyes or whatever else, the outline of a cube vs sphere and observe that only one has corners. The act of tracing should match the tactile sensation of moving your fingers around similar objects.