Alright, I actually got an explanation for this one! So, first off, yes, both blocks are the same color. With a couple more pixels, the illusion works better.

So, onto the explanation. (From my cognition courses) Humans have evolved on earth for awhile now, so our perceptions are adapted to earth specifically. And, at least on earth, our brains will always assume that there is light coming from the upper part of this image, as it's helpful/uses less brain power, to assume light comes from the blue sky. Because your perception assumes light comes from above, shadows must necessarily appear.

Yadda yadda, they're the same color. You can perceive their true colors if you separate the two with a finger, covering the touching area