[Zageron]

The answer is in this post, be careful if you want to solve it yourself.

A can't see anything. Doesn't speak.
B can't see anything, doesn't speak.
C Can only see a white hat, doesn't know.
D Can see both white and black, so doesn't speak.

Because D doesn't speak, C assumes that since B's hat is white, his is black.

Boom. (Don't forget to use all of the information. :P)