In order for a chicken to be a marans, according to the standard, they must lay a dark egg of a 4 rating or better (egg chart can be ordered from the marans club).
With that being said, consistent darkness of eggs the whole season is not typical. Usually they start out sporadic, then get a consistent color and often get lighter from there. At least that's what I've seen with the dark laying marans I have.
I only pick eggs to judge color after the pullet has been laying a consistent egg for three weeks or so.
Good luck with your birds.