BCM is what kind of Marans?
Black or Blue? Copper or Cuckoo?
I don't have any personal experience with that cross, but if the hens are Black Copper Marans, you will probably get chicks that look like Black Copper Marans.
If the hens are Black Cuckoo Marans, then the chicks will be sexlinks: sons will have a light dot on top of their heads when they hatch, daughters will not. Daughters will grow up to be solid black, possibly with some gold or red leakage. Sons will grow up to be black with white barring, possibly with some leakage of other colors too. The barring gene in the males is what makes the light dot on their heads when they hatch, as well as the white bars in their feathers as they grow.
Blue Copper hens will produce chicks just like what Black Copper hens produce, except that half the chicks will show blue instead of black.
Blue Cuckoo hens will produce chicks just like what Black Cuckoo hens produce, except that half the chicks will show blue instead of black.
No matter which kind of Marans you have, the chicks will probably have white legs (not the yellow of the Welsummer). If the Marans have some feathers on their feet, the chicks might have that too.
Daughters should lay eggs that are dark brown (probably between Welsummer color eggs and Marans color eggs.) I've heard that Welsummers are prone to laying eggs with dark speckles on them, but I don't know enough about the genetics of that to make predictions for the chicks.
