Oh, that's because it's happening in your retina. The reverse-video image is saturating the opposite-colour cones, so that when you stare at the B&W the whites and greys disproportionnately trigger the correct colour cones. Moving your eyes stops the illusion working because the overlay is now wrong and treated as 'noise' rather than sense by your brain.
Yes, it's obvious (to me, anyway) that that's how the basic illusion works. What I'm wondering is why it went black and white and then reverted to colour when I blinked.
At a guess, blinking de-saturates your rods, which are fully saturated in normal daylight, so that the B&W brightness information they're providing overrules the odd-coloured cone information temporarily, until they re-saturate a few seconds later, and the cone-illusion takes over again. Cones are much slower to saturate and to recover than rods.
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