By what the OP posted and my own experience as 20+ years breeder of French Marans and La Fleche, I suppose it is one of the young pullets/hens that laid this egg. Shell colour and egg form hint to it.To be clear: this hen started out laying a light-colored egg. Regardless of the genetics of the rooster she mates with, she will always lay a light-colored egg; his genes won't affect her eggs. If her eggs change color, some other factor is at play. Even if she is bred by a French Black Copper Marans rooster, a breed whose hens lay deep dark chocolate eggs, that's not going to change the color of her eggs.
An old hen nearing the end of her laying circle would have a different egg shape and shell quality as well as rather lighter than her usual egg colour than suddenly such an even dark tone.