Either they are not aware of what they were sold which are crosses or they aren't being truthful about only having BCM and AMs. I would bet they just do not know they have crosses. There are very few good breeders with decent birds and Noone breeding them is able to produce really good offspring all of the time. The breed is still so new in the US and there have been so many that do not breed to the standard therefore not improving the stock we have here in the US. Even 2 really good examples of the breed will produce not good offspring! It's an uphill battle for the Marans!
Just my experience and what knowledge I've gained with discussions within the USA Marans Chicken Club