There are sometimes blocks looks like they are on light level ~0. They are usually alone, sometimes in rows. Tihs is still happening in a superflat world in full daylight. The more interesting is that they are disappearing, if I'm nearby. This looks like ~ if I'm on the same, chunk, they disappear, but if I'm a bit further they sometimes just come back.
The most of the times just my horizontal distance matters, not the vertical.