One thing all Marans have, is white skin. Not all lay super dark eggs either. She has enough going for her that as long as she has white skin, I'd call her a Marans.
Is this the dark egg? Looks like a normal brown egg.
I'd say the bird on the right is a Marans mix. I don't think the one on the left it. I suspect that one has yellow skin.
BCMs should have some amount of red in their head/hackles.
The one on the left may be a Marans mix. What color are the soles of the feet on the bird on the right?