Cute chickies!
Sorry I misread your original post and somehow was thinking that you were referring to the blue as being the throwback. Sometimes, I have moments.
I am going to say (without know the genetic background of the birds) the "throwback" chick is a Splash. Splash is also blue, but carries 2 copies of the blue gene which leaves it looking white compared to its blue sibling that only carries 1 copy of the blue gene. Splash birds can be produced from the matings of 2 blue birds together and or a blue bird to a splash.
If this chick looks yellow and is not white-ish then I would say that it was a Wheaten. Some lines of Marans throw Wheatens. As the chick nears approx. 1 week to 10 days old, if it is a Wheaten you will be able to see sexable feather markings on the wings. If it is a Splash you should begin to see some random blue feathers coming in on the body. I doubt it is a Wheaten, my money is on it being a Splash.
Post photos of them in a week or so and someone will be able to help you out more.