Request pics of the eggs along with the parents. Lavender is a newer color in Marans and it is possible the egg color was diluted significantly when introducing that gene from another breed.
Don't rule out Welsummers. I have both BCM's and Welsummers and their eggs are identical to my eye. There are many strains of both with lighter than desired eggs, but also the dark egg laying strains of both breeds. I find Welsummers to be slightly better layers and a lighter bird. I'm also very enamored with the patterning of the Welsummer hens. The roos are gorgeous too. Not that BCM's are not pretty, just if I had to choose, I'd keep the Welsummers.
For a good background and comparison, look up Whitmore Farm and read about both. My strains of both came from there and I'm very happy with them.