What you are describing would be a Cuckoo Marans, or an Olive Egger if she lays olive eggs and has a tiny crest (though I did not see you mention this trait, so I assume she does not have a crest, and of course at this age you would not be getting eggs from her yet). Feed stores, especially Tractor Supply, incorrectly place their chicks constantly, switching breeds up and even species because of the common lack of knowledge in employees. I would not be surprised that your chick was not the breed she was labeled to be. Chicks sometimes fly out of the bins themselves and land in another. When there is not any room for extra chicks, the employees will stick them in a random bin of their choosing.
I'll second that with purchasing 4 Columbian Wyandottes and growing out 4 Cornish Cross cockerels.
