Oh and Black Copper Marans are the really dark layers, Cuckoo Marans aren't known for laying a dark egg unless you get get them from a Breeder who breeds for it. You would have to be pretty lucky to get a dark layer from a hatchery.
And ... as much as I've read in the BCM threads, the eggs are colored in a cycle not seasonally. The darkest egg is laid at the beginning of the cycle and it fades as the cycle progresses. Then she takes some time off and when she begins another cycle, the eggs are dark again.