What happens is this. Because of the arrangement of the nanotubes, light gets trapped in between them. Once it’s in there, it bounces around. So rather than reflecting back to your eyes, the light stays between the structures. And that creates the weird effect that Vantablack is most famed for.
Human eyes work by processing the light that reflects to them from objects. But Vantablack reflects almost no light whatsoever. As a result, we’re unable to properly understand what we’re seeing. Consequently, coating an object with Vantablack can have some peculiar effects that don’t seem to make any sense to us.