I agree they could very well be Cuckoo Marans....they have the Marans body shape (heavier, more bowl shape) and light grey-ish legs with messier looking black/white pattern rather than crisp barring (cuckoo vs. barred), and a single comb.
vs....Barred Rock would have crisp barring, not messy disorganized pattern, with yellow legs, often black wash down front of leg, single comb
vs....Dominique would have crisp barring, yellow legs, rose comb
As to Marans egg color, there is no guarantee as to how dark as each hen's genetics will determine the color she lays, and that can vary within the breed especially between lines...
As to whether they will stay dark, it helps to understand the process. In a brown egg, a brown wash is laid over white egg shell...literally painting the color onto the egg as it goes down the egg duct The longer it takes for the egg to go through, and some other genetics involved, the darker the egg. Thus dark brown layers lay dark eggs but usually a lot less prolific.Their darkest eggs are at the beginning of the season when the "wash" pigment is plentiful, but over the season the color can lighten considerably as the bird's reserves are more depleted.
Cuckoo Marans are not typically on the darker Marans scale...the Black Copper lays the darkest on the Marans scale. Carefully bred birds from breeders lay darker on the Marans scale. Hatchery stock often do not produce the really dark colors...just a deeper brown than the average brown layer.
So how dark your particular birds may lay depend a lot in their genetic background. Since you are not sure of their breed, I suspicion you got them from a feed store or other back yard source...Marans breeders know their lines and will show you their eggs as compared to the Marans scale...usually proudly displayed on their website. (I have a 5 scale Marans and just purchased a pullet whose parents were on the 7 scale).
Marans overall do not mature really early, so the actual age at 6 to 7 months is about right to begin lay. However, during this time of year,the shortening days are working against you. Maturing pullets need at least 12 hours of continuous daylight to stimulate the pituitary gland to stimulate the ovary to produce eggs. To lay consistently, 14 to 16 continuous daylight hours are needed. In our shortening fall hours, your birds may not get enough light to bring about laying until January, or so, depending upon where you are by way of parallel...it also depends on how bright your general area is (dark coop in shady area of yard) and the genetics of the birds. Commercial birds tend to lay earlier, having been genetically selected for early maturing....also commercial industry uses artificial lighting. You could add lights to the early daylight hours to lengthen the day to hasten laying, but it may take a few weeks to make a difference.
Or be patient...you may be surprised to find an egg sometime soon...or not until early Spring when daylight is naturally longer.
FYI: Edited to Add the Marans egg scale chart (and some clarity in breed id to answer OP's question of "how do I know")
Edited by Lady of McCamley - 10/13/15 at 5:25pm