Black birchen Marans
I honestly can't tell you for sure what you would get by breeding him to a fbcm but to get more black birchens you would ldeally need to breed him with other birchen's either blue, black or splash birchens. I think if you mix the birchen with the bcm then you would get more bcm chicks then anything and may lose the birchen gene. There are other way's don't get me wrong but unless you know what all gene's you have in your birds other way's are much harder. I had a pair of blue birchens last year that I lost was only able to get 1 hen from there last egg. happened to be a black birchen hen. I do plan my self to put her over my blue maran rooster. Because very small chance she could still produce some more birchen baby's or atleast ones with that birchen that will carry that birchen gene. I know others that work more with birchens could have some more advice.
My birchen pair did breed true, which is why I was so devisated when they got attacked and passed. I did manage to get one off spring. I to love the color they are very pretty. I actually got rid of all my black coppers to keep my birchens only have so much room. Me personally I like wheatens, pure blue, black and splash, birchens blue, black and splash. The best all though lavender would be pretty neat also.
Jon Burton in Vale, NC has birchens. I know he sales eggs on Ebay. I've never bought from him. There is one BYCer that has birchens, that's were I got my chicks. Her user name is Chuckeychickens. I know she was moving last year, so I'm not sure she still has her birchens. Nice sized, good feather, just got my first eggs from my pullets and got nice color. CC was in Chuckey, TN when I drove down.
Hope you find what you're looking for. Good luck.