My CM hens are really dark also compared to Belle. You said that it was an extra. Could it be a different breed all together? They rarely send extra pullets especially ones that they can sex by the white spot on their heads as chicks. The extras are their way of getting rid of extra roosters that they have not sold. Also agree with BeulahBreezes post about the parents. I have 3 different lines of Black Copper Marans and each line has their own distinct differences. Hope this helps.