You will have to call the hatchery mypetchicken and find out who supplied those Maran eggs to them. If they dont give you the name and excuses like that, then it is probably most likely they are hatchery bred birds ... I have seen some of the hatchery customers saying that their Marans egg shell color is very much like the Barred Rocks and sometimes, hatcheries DO put in Barred Rocks in place of Marans but these kind of cases are rare but they DO happened. You do have to ask alot of questions if you want to dig up where those birds came from.
If you are attached to your pullet, just wait and see what color of egg she will lay and then you will decide whether or not she is of dark egg layer.
I had bought some Cuckoo marans from Ideal hatchery and was VERY disappointed with the egg color that I sold them as Barred Rock pullets. I once had a Black Maran pullet I got from a BYC member that laid
very chocolate eggs and boy, that was a treat.
Best bet is to buy from reputible breeders and some of us in here got some very good stock. Sorry I dont raise Marans. All I know that Pasochicken got some Penescenda eggs but she only has a trio. I am expecting some of the eggs from her in a few weeks.