In general the Cuckoo Marans lay a lighter colored egg than the Black Copper or the Wheaten, but if the Cuckoo is of good breeding, from lines that have been managed to promote dark eggs, the color can be quite nice.
I am sure you will hear from Cuckoo breeders that will tell you they have eggs in the 7 or 8 range from Cuckoo hens. It is possible that your first egg is lighter than subsequent eggs, but again in general the darker eggs are earliest in the laying cycle.
I have a few Cuckoo (supposedly) from hatchery birds, the eggs they lay are no darker than the eggs from my RIR or any other breed. I think it quite possible that you have a Cuckoo Marans that is simply not going to lay very dark. As a matter of fact this reality in Marans and hatchery stock is where I really became interested in seeking out birds that have the genetics to get me where I want to be.
I hope I am wrong and that your eggs get darker as time goes on, but I would not bet on it.
The speckling may go away and you will get some eggs where the dappling is more equally distributed. I got both from my birds. I did have some Wellsummers that consistently laid speckled eggs I thought they were very nice looking.