While this is typically true.... it is not always. My Vanay, a proven charcoal, had light eyes.
And anery A can have dark eyes as well.
The key difference is level of contrast. Charcoals are low-contrast snakes. The saddle color and ground color are much more similar than the stark dark-light of a young anery.
For example, this is one of last year's charcoal hatchlings at 6 months old.
Adorable
Orpheus, 1.0 Normal Ball Python, born approx 2007 (?)
Zillah, 0.1 BCI (b 2011, d 2014)
Ruska 1.0 BCI (b 2011)
Zmija 0.0.1 Anerythristic Corn Snake (b 2013)
Otto 1.0 Bumblebee Ball Python (b 2012)
Spot 1.0 Western Hognose (b 2014)
Pork Chop 1.0 Western Hognose (b 2013)
Ophelia 0.1.0 Desert Ball Python (b 2012)