If you create a dark tunnel, as the length approaches 8 chunks, the far off blocks will glow as if they are in daylight. You have to be close to 7-8 chunks away in order to see this effect.

Examine the image, below. The blue blocks in the back are in total darkness, if you are close. But if you recede, you will see them illuminate blue, as if they are in sunlight. I have replicated this a few more times in various long tunnels.