i went thru this too, still actually. i bought a trio of cuckoo marans and they hadnt started laying yet. i was told they were under a year old so im not sure if they had laid before or not. one hen started laying first and the eggs were huge and not that dark, i was so mad. some days they were darker than others but nothing like what i wanted. then i started getting two eggs and one was SUPER dark. i to this day dont know if it was the second hen that started to lay or the first and her egg just got darker. all i know is one still lays much darker than the other. the lighter egg is darker than that of a RIR but im still not happy about it. i wanted very dark and was sold one so-so hen. all i can do now is hatch out some from my darker egg layer and keep a few pullets to see how they lay, then get rid of my lighter layer. both girls lay a huge egg tho! give her a couple weeks, it may darken because even my lighter layer has darkened some over time.
good luck i hope it was a fluke!