This figure is called the Hermann grid after L. Herman (1870). The dark blobs can be explained by reference to receptive fields and lateral inhibition.
Just in case you think you are being fooled, try taking two pieces of paper and cover all but two vertical or horizontal rows of black squares. Do you still see the dark blobs when viewing only part of the Hermann grid?
There is a reasonable explanation for this phenomenon. If you haven't yet looked at the receptive field story you should look at it or else the explanation for the Hermann grid won't make a lot of sense to you.
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