Hen suddenly changes to laying a dark egg

[LaFleche]

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.

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.

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.

 

[LaFleche]

Or...

Someone put it in your nest box as a joke and none of your own hens laid it.

 

[PoopCoopMama89]

Could we get a picture of the difference in color?
Have read the color doesn't change although to bloom does, so maybe she being older the bloom is now thicker? Just guessing, waiting for others to reply. You wouldn't happen the have any of these in your flock ... Welsummers, Barnevelders, Penedesencas, and Marans

First thing I thought of was my two Welsummers that I just got last week lol. Im a new chicken mommy myself and am intrigued by your response to look up Penedesencas, Ive yet to run across the breed online or otherwise.